






Glass \ K 0 _ 

Book_ 

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COPYRIGHT DEPOSIT. 













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UNDERWRITERS’ REQUIRE- 



A PRACTICAL DETAILED INTERPRETATION OF THE 
ELECTRIC CODE OF THE NATIONAL BOARD 
OF FIRE UNDERWRITERS 


By DANA PIERCE, B. A. 

1 

ELECTRICAL ENGINEER, UNDERWRITERS’ LABORATORIES, INC. 


ILLUSTRATED 


CHICAGO 

AMERICAN SCHOOL OF CORRESPONDENCE 
( l 1913 





T \< ^ fe o 

A(d 


Copyright, 1911, 1913 by 
American School of Correspondence 


Copyrighted in Great Britain 
All Rights Reserved 




©Cl. A3 500 5 4 

tu/ 



CONTENTS 


PAGE 

Introduction. \ 

Electricity as a cause of fires. 3 

Elementary electrical ideas and terms. 4 

Essential parts of electric installations. ' 17 

National electrical code. 19 

Power stations and their equipment. 20 

Generators. 21 

Switchboards. 25 

Resistance boxes or rheostats. 28 

Lightning arresters. 28 

Ground detectors and tests. 30 

Motors. 31 

Storage batteries. 41 

Transformers. 43 

Outside work. 43 

Wiring. 44 

Electrolysis. 49 

High tension lines. 51 

Mounting of transformers. 53 

Grounding of circuits. 53 

Inside work. 59 

Wiring systems. 59 

General rules on wires. 62 

Constant current systems. 64 

Constant potential systems. 67 

Installation rules for controlling and protecting devices... 68 

Switches. 68 

Fuses and circuit breakers. 71 

Electric heaters. 77 

Fixtures and fixture wiring. 80 

Fixture details. 80 

Sockets and receptacles. 82 

Flexible cords. 85 

Arc lamps on constant potential circuits. 88 

Transformers in buildings. 90 





































CONTENTS 


PAGE 

Installation of wires in buildings. 91 

Classification and general principles. 92 

Open work in dry places. 93 

Open wiring in damp places. 97 

Wires in molding. 101 

Concealed work. 104 

Armored cable. Ill 

Conduit work. 112 

Special installations. 124 

Decorative and commercial lighting. 124 

Decorative lighting. 124 

Electric signs . 126 

Theater wiring. 128 

General specifications. 128 

Dimmers. 129 

Footlights and borders....,.. 130 

Stage pockets. 131 

Special lighting circuits and stage effects. 133 

Requirements for stage auditoriums. 133 

Moving picture theaters and machines. 133 

Interior equipment. 133 

Causes of danger. 134 

Car wiring. 136 

Lighting and power from railway wires. 137 

High and extra high potential systems. 137 

Classifications. 137 

Requirements for safety. 138 

Signaling systems. 140 

Wiring requirements. 140 

Protecting devices. 143 

Testing. 144 

Voltmeter method. 145 

Devices and material. 148 

Rubber-covered wire. 149 

General specifications. 149 

Special insulation. 151 

Fixture wire. 151 

Insulation for conduit and armored circuits. 152 

Rigid conduit and conduit fittings. 152 








































CONTENTS 


Devices and material 

Rigid conduit and conduit fittings PAGB 

Unlined steel conduit. 152 

Conduit fittings. 153 

Fuses or cut-outs. 154 

Classification. 154 

Plug-fuses. 156 

Cartridge fuses. 157 

Knife switches. 159 

Snap switches. 161 

Circuit breakers. 163 

Classification. 164 

Miscellaneous. 165 

Panel boards and cabinets. 165 

Sockets and receptacles. 166 

Rosettes. 168 

Bell-ringing transformers. 168 

Heating devices. 169 

Electric gas lighters. 170 

Marine work. 171 





















AN UNUSUAL PICTURE OF A NIGHT FIRE AT DOVER, NEW HAMPSHIRE 

This Picture Was Ta'ken by the Light of the Fire a Few Minutes Before the Walls Fell 

















INTRODUCTION 


r jpHE National Board of Fire Underwriters, in order to stand¬ 
ardize electric wiring practice in the United States, established 
a “National Electric Code” in 1897. Previous to this time the 
specifications as to the kind of electric fittings to be used and the 
methods of running the wires in buildings or in overhead or conduit 
work had been determined almost entirely by the municipal regu¬ 
lations existing in the locality where the material was placed. 
Some manufacturers were making their fittings as well as they 
knew how, while others used cheap material and bad methods and 
disposed of their product, too, for there was no well organized 
attempt on the part of those interested in the quality of the work 
to lay down definite specifications which must be met. 

If As a result of these shortcomings in manufacture and inspection, 
such things as the overloading pf circuits, the deterioration of 
materials with time and use, the breaking down of poor insulation, 
and many other sources of trouble caused numerous disastrous 
fires with their accompanying losses which could easily have been 
avoided. Through the agencies of the “National Code” and the 
Underwriters’ Laboratories where the various manufactured devices 
are tested and approved, these evils have been corrected and 
the electric wiring installation work has been completely stand¬ 
ardized. The fire insurance companies, by demanding that all 
specifications be drawn according to the Code, have been able to 
bring about a uniformity of practice, a quality of workmanship, 
and a degree of safety which probably could not have been accom¬ 
plished in any other way. 

If The author of this work, by virtue of his connection with the 
Underwriters’ Laboratories, speaks with exceptional authority. 


INTRODUCTION 


By his comments upon the most important points he has clarified 
the rules of the Code, and shown the reasons for their adoption. 
By the use of copious examples and photographic illustrations, he 
has made the work, when used in connection with the Code, a 
perfect guide to the best practice for electric wiring installations 
of all kinds. It is the hope of the publishers that the work will 
be of exceptional value to electric wiremen, engineers and fire 
insurance inspectors as well as of interest to the general reader. 







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UNDERWRITERS’ LABORATORIES, CHICAGO 



















































































UNDERWRITERS' REQUIRE- 
MENTS 

PART I 


INTRODUCTION 

The insurance interests are concerned with electric work only 
for the reason that such work if done improperly constitutes a fire 
hazard, and underwriters’ rules on electrical installations are, there¬ 
fore, confined to such questions as concern proper methods and 
materials to be employed to minimize the chance of fire arising from 
the use of electricity for light, heat, and power. Electricity as 
employed for signaling work, such as telegraphs, telephones, call 
bells, burglar alarms, and similar purposes, is not covered by in¬ 
surance rules except in so far as such installations may become dan¬ 
gerous because of the liability of wires in such systems becoming 
crossed with electric light, heat, or power circuits. 

The many applications of electricity for municipal fire-alarm 
systems, factory or isolated plant fire alarms, watchman’s time¬ 
recording appliances, and automatic alarms employing electric 
thermostats, are not covered by the general rules for electric work. 
They do not in general tend to cause a fire hazard of themselves 
but come rather under the head of protective devices and the dis¬ 
cussion of them does not, therefore, come within the scope of this 
paper. 

The earlier installations of electrical lighting and power ap¬ 
pliances were very crude and were often made with no considera¬ 
tion of what are now generally admitted to be questions of great 
importance from the viewpoint of fire protection. It was natural 
that experience should have been necessary to demonstrate the 
need of protection against unreliable or hazardous methods of apply¬ 
ing electricity and to develop improved materials and devices. 
The comparatively mysterious nature of electricity as viewed by 
the general public led at first to a habit, not yet wholly outgrown, 


Copyright , 1912 , by American School of Correspondence. 



2 


UNDERWRITERS’ REQUIREMENTS 


of attributing to electrical causes all fires for which no other cause 
was readily apparent. Today, however, the production of light 
and the transmission of power are without question accomplished 
more safely as well as more conveniently and economically by 
electricity than by any other means. 

In judging electrical work as affecting the fire hazard, the fire- 
protection engineer and underwriters cannot undertake to do for 
the assured, the work which lies within the province of the electrical 
engineer on whom properly falls the responsibility of designing 
installations and choosing the methods and materials to be used 
to accomplish the results desired. In electrical, as in other engineer¬ 
ing, consideration of first cost, economy of operation and main¬ 
tenance/efficiency of machinery and appliances, depreciation, and 
reliability are of prime importance. It is the proper function of the 
fire-protection engineer to act as critic of the plans and their execu¬ 
tion in order that other considerations involved shall not be allowed 
to dictate methods which do not afford suitable and reasonably 
safe fire conditions. Electrical engineering is far too complicated 
a profession to be fully mastered by the fire-protection engineer 
or by an inspector, but the general principles of safe electrical work 
can be mastered and applied without presuming to encroach on the 
work of either the consulting or the installing engineers. 

Architects are now very generally prepared to recognize the 
necessity of providing in their plans for the electrical installations 
which in more elaborate buildings must be given very careful atten¬ 
tion if adequate provision is to be made for electric wires and ap¬ 
pliances. It is in the smaller and less carefully planned structures 
that the electric work is most likely to be left to the installers to be 
put in as best it may. A well considered plan adapted to the type 
of building and the uses to be made of current is an essential to 
successful and safe work. Makeshifts are usually dangerous. For 
this reason, also, it is often much more difficult to secure proper 
electrical work in wiring old buildings than new ones. 

Electrical contractors are usually willing to do good work for 
a fair price and most poor work is due to an attempt to secure busi¬ 
ness at too low rates in order to meet competition. Since a faulty 
or dangerous electrical installation may perform its appointed work, 
and since electrical defects are not easily discovered, often develop- 


UNDERWRITERS’ REQUIREMENTS 


3 


ing after some time, only thoroughly good work from the start 
affords real protection against disastrous failure from causes which, 
after a fire, may be very difficult to assign but which might have 
been easily foreseen and removed. 

Electrical inspectors either of the underwriters or of the munic¬ 
ipality should be fully acquainted with approved methods, rules, 
reasons for rules* and must have a great range of practical knowl¬ 
edge which only actual field experience can give; and such inspectors 
must be specialists of a high order. The fire-protection engineer 
must refer to such specialists the technical details while undertaking 
himself to be a competent judge of general methods and principles. 

Electricity as a Cause of Fires. Electricity may in general be 
the cause of a fire in either one or both of the following ways: 

First: By causing wires, cables or other conductors to be 
overheated by the passage of the “electric current,” thereby setting 
fire to the insulations on such conductors or to nearby combustible 
materials. All conductors are heated by any current however small, 
but if the conductor is badly overloaded it may become red- or 
even white-hot. Hence it becomes necessary to prescribe the safe 
carrying-capacity of wires, and all conducting parts of electric 
appliances must be properly proportioned. Such parts as are pecu¬ 
liarly liable to become heated or wiiich must be heated to perform 
their proper function must be suitably protected and separated 
under all circumstances from materials wiiich might be ignited. 

Fuses, circuit breakers, and other automatic appliances have 
to be installed to afford protection in case of accidents which may 
result in conductors or other devices being overheated by abnormally 
large currents. Such protective devices may themselves become 
dangerous and the rules, therefore, prescribe carefully how they 
must be constructed and installed. Overheating of conductors is 
thus one of the tw~o general w^ays in which electricity may cause a 
fire and provisions against such accidents, therefore, form an im¬ 
portant part of the rules. 

Second: Electricity may form “arcs.” An arc is the visible 
evidence that the current is passing from one conductor to another 
or across a break or gap through the air or over the surface of an 
insulating material. An arc always causes heat and if any appreciable 
current passes, the arc w ill be very hot and if continued is capable 


4 


UNDERWRITERS’ REQUIREMENTS 


of melting the adjacent metal at the gap or setting fire to any com¬ 
bustible matter near by. It, therefore, becomes necessary to pre¬ 
scribe rules, the observance of which tends to lessen the chance of 
accidental arcs being established, and to so locate all appliances, the 
operation of which produces arcs, that no harm will result. Switches, 
circuit breakers, and fuses are all “arcing” devices. The problem 
of insulation, spacing between conductors and excellence in materials, 
methods of installation and workmanship, all have to do chiefly 
with the prevention of accidental or dangerous arcs. 

From the viewpoint of fire hazard, it is well to treat all con¬ 
ductors, however well insulated, as bare, and to proceed to furnish 
adequate protection against grounds or short-circuits on that basis. 

In judging of installations it must constantly be kept in mind 
that conditions are liable to become worse rather than better after 
the wiring and appliances have been in use for some time. Require¬ 
ments are, therefore, made to anticipate in part such deterioration 
as is inevitable in even the best equipments, and which may be sur¬ 
prisingly rapid when inferior materials are put in by careless work¬ 
men and used and abused by those having little or no understanding 
of electrical affairs and no appreciation of the hazards involved. 

The following are the chief general requisites for a safe electrical 
installation: Excellence of material; simplicity in design so far as 
compatible with the results to be secured; ease of inspection and 
repairs of all wiring and appliances; thoroughly good mechanical 
execution of the work; the choice wherever possible of the more 
protected and safer forms of wiring; and the use of “approved 
fittings.” No rules can take the place of good designing nor will 
perfunctory obedience of rules make a poorly executed job, safe. 
The architect, the owner, the manufacturer of devices and materials, 
the electrical contractor, the electricians who install and those who 
operate the plant—each and all must share the responsibility with 
the insurance and the municipal inspector. 

Elementary Electrical Ideas and Terms. Electric power may 
be transmitted by either direct or alternating current. 

Direct Current Direct current is current of such character 
that what is usually called the “direction” of the current is always 
the same, or, more exactly, the magnetic effects of the current are 
not being reversed from instant to instant. If a small compass be 


UNDERWRITERS’ REQUIREMENTS 


5 


held near a wire carrying direct current, the needle may be caused 
to turn away from its natural north and south line. Thus in Fig. 
1, if D is a direct-current dynamo connected to a wire from south to 
north, a compass needle over the wire, before the switch T is closed, 
will point along the wire; when the switch is closed, if current flows 
as shown by the small arrows on the wire, the needle will turn as 
shown. The amount it will turn is an indication of the amount of 
current, but the needle will remain stationary in its new position 
if the current is direct current. A battery gives direct current and 
so does a direct-current dynamo. 

Alternating Current. If, however, the current came from an 
alternating-current dynamo or from a transformer supplied by such 
a dynamo, the needle would tend'to swing very rapidly first to one 
side of the wire, and then to the other. This would also be the case 
if the connections on the 


direct-current machine, Fig. 

1, were rapidly exchanged 
back and forth. Such re- ^ 
versals of current direction 
are made automatically by 
an alternating-current dy- z' 1 
namo. The number of Vi 

T 


T 

. 

(3) 

) 

d 

(b) 

changes per second is called 
the frequency and 25, 60, 
and 135 are the commonest 

—- ZfzTY* - 


— /fzTV— — 


pnmmprrial frenneneies of Fi S- l - Diagram of Simple Dynamo Circuit 

commercial liequeucieb U1 Showing Effect on Magnetic Needles 

alternators. Evidently no 

compass needle could actually vibrate so fast, but it tends to do so, 
and the result is that the needle does not appear to be affected by 
the alternating current. 

Both direct- and alternating-current systems are in common use 
for light and power. Street railways use at present chiefly direct- 
current systems. Where power must be conveyed to considerable 
distances, alternating current is used because it is more economical 
under these conditions. Different motors and somewhat different arc 
lamps are required for direct and for alternating circuits. Incandes¬ 
cent lamps are the same for either system as are also most heating de¬ 
vices. Transformers can be used only on alternating-current systems. 










6 


UNDERWRITERS’ REQUIREMENTS 


The distinction between direct and alternating current is not 
one which has many important consequences in the safeguarding of 
electrical work. There are, however, a few important cases where 
different rules are established. It should be remembered that direct 
and alternating currents of the same strength produce the same 
heating effect in a given conductor. In some cases, however, with 
alternating currents an additional heat effect is produced in certain 
appliances by the magnetic action of iron cores, of coils, or other parts 
of the apparatus. In general, alternating current produces less 
severe and persistent arcs than direct current of the same strength 
and voltage. Furthermore, alternating-current motors in particular 
are somewhat less liable to emit sparks and, therefore, have a certain 
advantage. As a whole, however, no distinction may be made as 
regards fire hazard between direct- and alternating-current installa¬ 
tions which should be made with the same care in workmanship 
and with the same precautions as to insulations, fuses, and all pro¬ 
tective devices. 

Current. Current is measured in amperes. It may be com¬ 
pared to the number of “gallons per minute” carried by a water pipe 
through which a stream is flowing. More “current” will, other 
conditions being equal, do more work, and will always cause more 
heat in the conducting wires and cables and in the appliances, lamps, 
heaters, resistances, motors and the like which use the current. - 
furthermore, the heating effect in conductors such as metals varies 
with the square of the current,” i. e., if one unit of current pro¬ 
duces a certain amount of heat in a w 7 ire, twice as much current w ill 
cause four times as much heat in the same wire, three times the current 
will cause nine times the heat and so on. An instrument for measur¬ 
ing current is called an ammeter. 

Note. The heat liberated is a measure of energy or power consumed 
in the wire. The temperature of the wire will not necessarily follow the rule 
given above, this being dependent not only upon the heat developed but 
also upon the surroundings of the wire as affecting the readiness with which 
the heat may be radiated or otherwise gotten rid of. 

Voltage. \ oltage or potential is measured in volts. Volts 
measure the propulsive force which causes “current” to flow 7 through 
a conductor. It may be thought of as an electric pressure produced 
by the dynamo, battery, or other generator of electricity. Through 


UNDERWRITERS’ REQUIREMENTS 


7 


a given circuit a higher voltage will in general cause a proportionally 
greater current. Since it is the voltage which may cause electricity 
to pass from its proper path and seek other and perhaps dangerous 
paths, higher voltages require better insulation on wires and in all 
electrical appliances. The voltage used thus becomes an important 
factor in determining the protection necessary for safety. 

Difference of Potential. Two points, as on conductors or be¬ 
tween a conductor and the earth, are said to be at different potentials. 
When such an electrical condition exists on them a current tends to 
pass between them either along the conductor or across a gap be¬ 
tween the points. Along the conductor such a current produces 
heat, while if the current “jumps the gap” or arcs, heat is produced 
in the spark or arc formed. 

Resistance. Resistance is measured in ohms. All substances 
offer resistance to the passage of current. This is true of metals, 
liquids, and gases. A good conductor, such as copper, has com¬ 
paratively little resistance. Other materials, such as slate, porce¬ 
lain, and rubber, are very poor conductors and may generally be 
considered as insulators. Since heat is always produced, the con¬ 
ductors must be of suitable size and material to keep the tempera¬ 
tures below the dangerous values, or, in cases where the heat is the 
result desired, suitable protection must be provided. The resistance 
of conductors is thus a necessary but undesirable property in some 
ways and a usable and valuable property in others. In all cases 
the fact that current produces heat in conductors must be reckoned 
with in electrical problems. 

The loss of power from the heat expended in a supply w T ire is an 
illustration of the undesired property of resistance. The electric 
flatiron is an appliance where the resistance produces a useful result. 

Ohm’s Law. For our purpose here the relation between cur¬ 
rent, voltage, and resistance in a direct-current circuit may be stated 
as follows: The current (amperes) in a circuit equals the voltage 

E 

(volts) divided by the resistance (ohms), or 1= — • This is true both 

R 

of entire circuits and parts of circuits, provided E and R are the 
voltage and resistance of the whole circuit or the part under con¬ 
sideration, respectively. For a. c. circuits a somewhat more elaborate 
formula must be used. 


8 


UNDERWRITERS’ REQUIREMENTS 


Power. Power is measured in watts. ' (A kilowatt is 1,000 watts.) 
In direct-current circuits the power expended in any portion of the 
circuit is obtained by multiplying the current in amperes by the 



voltage across the portion of the circuit. Thus: If the current in 
an incandescent lamp is \ ampere and if a voltmeter shows that the 
voltage across the lamp terminals is 110 volts, the power is 55 watts. 




Fig. 3. 


6 Amperes 



Series Arc-Lamp Circuits 


The corresponding mechanical term is horse-power; 1 horse-power 
is equivalent to 746 watts and 1 kilowatt equals about 1J horse¬ 
power. With alternating current the power in watts is not always 


SHUNT. 




Fig. 4. Diagram of Shunt Circuits 


to be obtained by multiplying amperes by volts. With alternating 
currents, a third factor must be used, called the “power factor” of 
































UNDERWRITERS’ REQUIREMENTS 


9 


the circuit. It is sufficient for our purpose here to remember that 
the power actually delivered and used with alternating current 
may be less than the simple product of amperes and volts. Thus, 



Fig. 5. Single-Pole Switch and Double-Pole Fuses 


if a current of 50 amperes as read by an ammeter, passes through 
a coil of wire, and the voltage across the terminals of the coil is 100 
volts, the pow r er consumed, if the current is direct, is 50X100=5,000 
'watts or 5 kilov r atts. If, how r ever, the coil surrounds an iron core 
(as in an electromagnet) and alternating current is used, the* power 
consumed will not be 5 kilowatts. The powder factor may be 60 
per cent for this coil, and the power consumed will then be 50X100X 
.60=3,000 w 7 atts or 3 kilowatts, instead of 5 kilowatts. The follow¬ 
ing definitions wdll be useful to those unfamiliar w ith electrical terms. 

Multiple Connection. When a number of devices such as lamps, 
motors, etc., are so connected that the current has a path through 



Fig. 6. Short Circuit in the Floor Above Chandelier 


each device separately from one supply wire to another, they are 
said to be connected in multiple. See Fig. 2. Incandescent lamps 
are almost always connected in multiple. In Fig. 2 if each of the 
7 lamps shown takes 1 ampere of current the total current at A 

































10 UNDERWRITERS’ REQUIREMENTS 

and at B will be 7 amperes, while at C and at D it will be 2 amperes. 

Series Connection. When a number of devices are so connected 
that they come one after another, they are said to be connected 
“in series.” In Fig. 3, the arc lamps are shown so connected. In 
this case the same current traverses all parts of the circuit and the 


total current is no more and no less than the current in each con¬ 
nected series device. 

Shunt. A shunt is a by-path between two points so connected 
that part of the current will traverse it. The division of the total 
current between the main path and the shunt will depend on the 
comparative resistances, the larger current going by the path of 
lower resistance. In Fig. 4 are shown the connections of a “shunt 
motor” and resistance box “shunted” by a wire. 


Fig. 7. Effects of a Short-Circuit in Wires Under Floor 









UNDERWRITERS’ REQUIREMENTS 


11 


Cut-Out. A cut-out is a device for automatically breaking a 
circuit, usually when the current reaches a predetermined value. 
Thus, a 60-ampere fuse is a cut-out designed to burn out when 
currents in excess of 60 amperes pass through it. A circuit breaker 
is an electro-mechanical switch which is also used as a cut-out. 

Switches, fuses and other appliances are said to be single-pole if 
they are for but one wire of a circuit; double-pole if for two, triple-pole 
if for three. Fig. 5 shows a single-pole switch and a double-pole fuse. 



Fig. 8. Arc Between Wires of 250-Volt Circuit 


Ground. A ground is a connection either intentional or accidental 
between a part of an electric circuit and the earth, or any metal or 
other conducting substances which are in electrical connection with 
the earth, such as water and gas pipes, iron beams, etc. 

Short-Circuit. A short-circuit is a connection which permits 
current to flow from one part of a circuit to another by any path 
which it is not intended it should take. Since such a connection is 
the result of accident or the failure of some insulation, and since it 
usually allows excessive currents to flow, a short-circuit may be 
very dangerous and liable to cause a fire, especially as the accidental 
connection may afford very poor contact and cause arcing and 
burning at the junction of the conductors. Fig. 6 illustrates a short- 
circuit in a floor between the two supply wires to a chandelier in- 













12 


UNDERWRITERS’ REQUIREMENTS 



stalled on the ceiling below. The effect of this short-circuit is shown 
in Fig. 7. Fig. 8 is from a photograph of an arc produced by a 250- 

TO 6ENEPAT0R 


Fig. 9. Diagram of a Constant-Potential System 

volt ampere current between two wires which were touched together 
at a point where the insulation had been destroyed. 

Constant-Potential System. A constant-potential system is one 
in which the voltage between the main supply wires is approximately 
the same at all points. Such a system is supplied by a constant- 


potential generator, which is of a character to furnish greater current 
as more devices, lamps, motors, etc., are put into use. Most in- 


Fig. 10. Series Arc-Lamp Generator 


















UNDERWRITERS’ REQUIREMENTS 


13 


candescent-lamp, motor- and street-railway circuits are of this type 
and also many arc-lamp circuits. Fig. 9 shows such a 110-volt 
system and if the distributing 
wires are properly chosen the 
voltmeters at the points A'B 
and C will all read approxi- + 
mately 110 volts. Devices on 
such currents are connected + 
in multiple. 

Constant-Current System. 

A constant-current system is 

. . . . Fig. 11. Diagram of a Three-Wire Circuit 

one in which the current is 

automatically maintained approximately the same regardless of the 
number and character of the power-consuming devices in use on the 
circuit. Devices, usually arc lamps, are connected in series and the gen¬ 
erator is of a type which automatically increases the voltage or electric 
pressure as more lamps are turned on. Fig. 3 shows such a circuit and 
Fig. 10 is a picture of a generator used for series arc-lamp work. 

Two-Wire System. A two-wire system is one having a single 
pair of distributing wires. 

Three-Wire System. A three-wire system is a special form of a 
multiple system usually employing two generators. Fig. 11 shows 
such a system with two-wire branches to incandescent lamps. 

Transformers. The chief advantage of alternating over direct 
current lies in the fact that electric power can by alternating 




current be transmitted at a high voltage from the generating station 
to the point where the power is to be used, and there transformed 
with small loss to a voltage better suited to motors and lamps. 

It is far more economical to transmit power at high voltages. 
It can be shown by electrical theory that by using twice as high 









































14 


UNDERWRITERS’ REQUIREMENTS 



voltage, a wire only one-fourth as large is needed to transmit a given 
amount of power with the same percentage of loss on the trans¬ 
mission line. It should be remembered that if the voltage is doubled 

the current need be only half 
as great in order to have the 
power delivered the same. 
But half as much current 
heats the wire which carries 
it only one-quarter as much 
and, since the heat dissipated 
on the wire is lost power, it 
is evident that economy re¬ 
quires that transmission of 
electric power be accomplish¬ 
ed by high voltage and rela¬ 
tively small currents rather 
than the reverse. Thus it 
may be commercially possi¬ 
ble to utilize a water power 
at a distance from a city to 
make high - voltage current 
which can economically be 
transmitted to the city over 
a line of rather small copper 
wires. It would never do, 
however, to carry this high- 
voltage current into buildings 

Fig. 13. Interior View of Incandescent- Oil aCCOUllt of the great fire 
Lighting Transformer . , . tip . . 

risk involved, tor the insula¬ 
tions in wiring cannot be made safe for high voltages. With direct 
current there is no very economical way to change a small current 
at high voltage into a larger current at a lower and safer voltage. 
With alternating current, however, this can be done very readily 
by means of a “transformer,” a device which consists in its simplest 
form of two entirely separate coils of wire wound on the same iron 
core; in fact, a sort of double-coil magnet. 

Fig. 12 shows two shapes which a transformer may take. In 
each diagram suppose an alternating current at 2,200 volts and 1 





UNDERWRITERS’ REQUIREMENTS 


15 



ampere is supplied to one coil of 100 turns. Then a current of 220 
volts and 10 amperes will be induced in the other coil which has but 
10 turns, that is, the voltage will be “stepped down” in the same 
ratio as the number of turns on the primary (power-supply) coil and 
secondary (power-using) coil, and if there were no power loss in the 
transformer itself, the current would be multiplied by the same ratio 
inverted. In the case illustrated there are one-tenth as many turns 
in the secondary coil as in the primary and, therefore, the secondary 

2200X1 . . 

voltage will be ———=220 volts, and the secondary current will be 

approximately ten amperes for every ampere supplied to the primary. 
If the power were supplied to 
the coil of the transformer 
having few turns, and used 
by the current induced in the 
coil having more turns, we 
should have a “step-up” in¬ 
stead of “step-down” trans¬ 
former. 

It should be especially 
noted that the action of a 
transformer depends wholly 
on the principle that the 
rapid changes and reversals 
of current in one coil will 
induce currents in the other 
coil. Alternating currents are 
rapidly and systematically 
changing and reversing cur¬ 
rents, and thus transformers 
can be used to change the voltage and currents in a. c. circuits. 
Direct currents do not change in amount and direction and, there¬ 
fore, transformers cannot be used at all on direct current. 

Figs. 13 and 14 show the interior and exterior views of a small 
transformer of a type used for incandescent lighting. The case is 
designed to be filled with oil. Transformers are constructed of all 
sizes up to very large units, and are cooled either by air, air blast, 
oil, or by forced circulation of oil through the windings and core. 


Fig. 14. Exterior View of Incandescent 
Lighting Transformer 















16 


UNDERWRITERS’ REQUIREMENTS 


Alternating-current dynamos and transformers are constructed 
and connected in several ways which differ in regard to the number 
of partially independent circuits which they supply. An a. c. circuit 
in which there are but two wires from the generator to the trans¬ 



former or to lamps or motors is called a single-phase circuit. Fig. 
15 shows the path of the current in a single-phase circuit which in¬ 
cludes a generator G, switchboard with ammeter A and voltmeter 
V, transformers TT supplying power to lamps L and single-phase 
motor M. Single-phase circuits are used for lighting, both incan¬ 
descent and arc, and for power. 

The other most common type of a. c. circuit is called three-phase , 
and is also used both for light and power. The essential parts of 
such a system are shown in Fig. 16 where it will be noted that the 
dynamo has three slip rings, the circuit has three wires and the 
transformer has three coils in the primary T i, and three in the sec¬ 
ondary r 2 . In a three-phase circuit each pair of wires carries what 
may be considered a separate alternating current which varies in 
each pair in regular cycles, so that the maximum current in each 



Fig. 16. Diagram of Three-Phase Alternating-Current Circuit 



phase or pair of wires is reached at different instants of time. Such 
circuits are very widely used, especially for power, with what are 
known as three-phase induction motors. 












































































UNDERWRITERS’ REQUIREMENTS 


17 


3 


Essential Parts of Electric Installations. For the general pur¬ 
poses of this book the essential parts of electric installations are as 
follows: 

Generators. Generators include all dynamos and storage 
batteries. These present the hazards inherent in all machines of 
any sort which produce or transform large amounts of power. 

Cables. Cables and wires used for transmitting electric power 
are of concern from a fire viewpoint because of the possibility of 
their becoming overheated by the current, and because failure of 
the means provided to confine the electric current to them, namely, 
insulation, may give rise to arcs, i. e., flames from powerful electric 
sparks or discharges. The rules for safe wiring, therefore, are de¬ 
signed to limit currents on wires 
to safe values and to insure in¬ 
tegrity and reliability in the in¬ 
sulations employed. 

Closely associated with the 
conducting wires and cables are 
those devices which serve to reg¬ 
ulate the amounts of current, to 
open and close electric circuits, 
or to change the characteristics 
of currents at or near the appa¬ 
ratus utilizing the power. Of 
this sort are rheostats, switches, 
wltage-regulators, motor-generator sets, rectifiers, and the very common 
alternating-curre t transformer. All these must have suitable insula¬ 
tion and mechanical protection, and there must be due protection 
from the heat or arcs which may be produced in them. 

Power-Consuming Devices. The utilization of electric power, 
as of all power, is always accompanied by the production and dis¬ 
sipation of heat. Power-consuming electric appliances such as 
motors; all lamps, both arc and incandescent; heaters; and every 
other appliance using electric current, are, therefore, heat producers 
to a greater or less extent and must be so treated by the fire-preven¬ 
tion engineer. 

Protective Devices. Protective devices are the safety valves of 
electric circuits. A fuse, Fig. 17, is a portion of an electric circuit 





Fig. 17. Forms of Fuses 


















18 


UNDERWRITERS’ REQUIREMENTS 


purposely made so that it will melt and open the circuit when the 
current rises to a value which creates dangerous conditions in other 
parts of the line. The proper design and use of fuses thus involves 
the two questions of suitable protection to the wires and appliances, 
and the liability of the fuse itself to cause a fire when it operates. 

Circuit Breakers. Circuit breakers, Fig. 18, are automatic 
switches arranged to open the circuit if currents reach too great 
values. They thus accomplish mechanically the same results as 
fuses. As a general principle it may be remembered that high- 



Fig. 18. Automatic Circuit Breaker 


voltage circuits are more hazardous than low-voltage circuits, be¬ 
cause they are more able to injure insulations and to produce arcs 
across gaps or from one wire to another. Large currents are not 
more dangerous than small currents so long as they have propor¬ 
tionally larger wires to carry them. In case of failure, however, 
large currents (many amperes) will cause more trouble. The per¬ 
sistency of an arc and the difficulty of suppressing when once started, 
are dependent upon both the voltage and the current (amperes), but 
it is the voltage which causes arcs to form, as across air gaps or by 
puncturing insulation on a cable. 








UNDERWRITERS’ REQUIREMENTS 


19 


The production, transmission, and use of energy are necessarily 
attended with certain hazards which are in a sense proportionate 
to the amount of energy employed. The peculiar property of elec¬ 
trical energy is that a failure or accident to electrical appliances 
often permits a large amount of energy to be suddenly expended in a 
very short space of time, as at an arc or short-circuit, and great heat 
is, therefore, developed. 

NATIONAL ELECTRICAL CODE 

As the result of the united efforts of insurance, electrical, archi¬ 
tectural, manufacturing, and allied interests, there has been de¬ 
veloped a comprehensive body of rules governing electrical work. 
The development and experience leading to these rules covered 
many years, but in 1897 what is known as the National Electrical 
Code*, was first issued by the National Board of Fire Underwriters 
with the approval of a large number of engineering associations of 
national scope, whose endorsement has given to these rules a stand¬ 
ing and reputation unequaled by that of any other body of rules on 
an engineering topic in this or other countries. 

The Code is generally admitted to represent the best opinion 
on electricity with relation to the fire hazard under the conditions 
existing in the United States. It is in general use by insurance 
companies, rating and inspection bureaus and departments, and 
engineering organizations. It is either adopted unchanged as the 
official body of rules for cities, or is, in some cases, used as the basis 
for the rules prescribed by the city government with a few changes 
or additions which are demanded by peculiar local conditions or 
which give greater protection against injury to persons than can 
be prescribed by insurance companies, which are, of course, not 
primarily concerned with the life hazard. These changes are, in 
nearly every case, of minor importance and consist usually of cer¬ 
tain more stringent requirements than have found acceptance as 
a part of the Code. Few, if any, city rules are less exacting than 
the Code and nearly all are identical with its provisions. 

It should be noted, however, that the Code does not attempt 
to prescribe the most efficient or economical means of applying 
electric currents, nor does it formulate engineering rules or practice, 

♦The National Electrical Code will hereafter be referred to as the “Code.” 



20 


UNDERWRITERS’ REQUIREMENTS 


except in relation to fire hazards. It is published every two years 
following a public meeting in New York City at which all proposed 
changes or additions are thoroughly discussed. These meetings are 
held under the direction of the Electrical Committee of the National 
Fire Protection Association, a society the active members of which 
are insurance boards and bureaus; electric and architectural associa¬ 
tions and institutes and other societies of national character. The 
Code has, therefore, the authority not only of underwriters, but also 
the indorsement of all the chief associations in the electrical and 
building industries. 

Twice every year (April and October) is issued the List of Elec- 
trical Fittings which gives, under the names of manufacturers, the 
devices which have been approved for use after samples have been 
examined and tested in accordance with the Construction Rules by 
Underwriters’ Laboratories, an institution located in Chicago, 
Illinois, and maintained by insurance interests for the purpose of 
making investigations having a bearing on the fire hazard. 

In the pages which follow, no attempt will be made to restate 
all of the rules of the Code, but rather to indicate the chief under¬ 
lying principles of the rules and explain some of the reasons for 
them. 

Copies of the Code can be obtained without cost from any 
insurance bureau and reference to the Code will be made freely in 
this book. It is, therefore, assumed that the reader has the Code 
before him and will refer to it for the official statement of the several 
rules, and for the relatively less important details not repeated in 
this text. 

POWER STATIONS AND THEIR EQUIPMENT 

There are some peculiar hazards present in stations or rooms 
used for generating electricity aside from those always involved 
where larger amounts of power are produced and controlled. 

In the case of companies furnishing electricity for light and 
power, the generating and transforming stations are often of large 
size, and contain apparatus which is very expensive and also of 
enormous importance for the uninterrupted maintenance of the 
service they render. Modern practice, therefore, tends toward the 


UNDER WRITERS' REQUIREMENTS 


21 


housing of such central plants in separate buildings of strictly fire¬ 
proof construction, and the employment of every resource of en¬ 
gineering skill and experience to prevent fire from causing loss, 
either by the destruction of the equipment, or by interruption of 
service resulting therefrom. 

The protection against fire in such stations thus becomes one 
of the chief considerations in the design of the whole plant and 
passes, therefore, in great measure into the field of the designing 
engineer. In any modern, well-designed central station the original 
equipment is usually fairly good from the viewpoint of fire hazard. 
The chief troubles often result from crowding equipment when it is 
found necessary to enlarge the capacity of the plant. Ample room 
for operating and for inspecting is absolutely essential. Accessibility 
of all portions of electrical equipment, the use of fireproof materialc 
throughout, the installation of protective devices of approved de¬ 
sign, and of ample capacity and arrangements, by which any trouble 
may be readily confined to a limited portion of the equipment, are 
chief requisites of a central-station equipment from the viewpoint 
of the electrical fire hazard. 

Standard methods of installing generators, switchboards, and 
all transforming devices, should be followed, but the engineering 
requirements of special cases and the complexity and variety of such 
equipments in present-day stations render it manifestly impossible 
to describe in definite terms the details of construction and installa¬ 
tion. In general, generators and motors of themselves present less 
hazard than the switchboards with the mass of wiring often found 
on them, the transformers with their charges of inflammable oil, or 
the large conductors carrying heavy currents and presenting large 
surfaces of combustible insulations. 

Strict compliance with all accepted rules for wiring is of as great 
importance in central stations as elsewhere, both because of the 
large values involved, and because of the large amounts of energy 
to be controlled on the circuits. Central stations, therefore, should 
never be considered as outside of “Code rules” so far as they apply. 

Generators. Generators for either central stations or isolated 
plants should always be located in a dry, light place as the presence 
of moisture is apt to injure the machines or may in extreme cases 
cause short-circuits in leads to the machine. No combustible ma- 


22 


UNDERWRITERS’ REQUIREMENTS 


terial should be permitted near a dynamo, and it is desirable that 
there should be good ventilation to maintain reasonably low tem¬ 
perature, thus increasing the capacity of the machine. 

The presence of generators in rooms where any hazardous 
process is carried on is not to be allowed and it should be remem¬ 
bered that sparks from generator brushes or rings may readily set 
fire to inflammable gases or to flyings of lint. Generators should 
be raised above the floor and should preferably be mounted on 
wood bases (Fig. 19) which insulate the machine frame. Such 
insulation will prevent a failure of insulation in the machine windings 



Fig. 19. Generator on Wooden Base 


from grounding the system. Such a ground might cause a short- 
circuit if the system became grounded at another point. If the frames 
are not insulated, a reliable ground connection is required in order 
to give a good path for any leak currents which may occur, instead 
of an accidental path which may be dangerous. 

In generating plants supplying current to railway power lines, 
such as trolley and third-rail equipments, the current usually is 
carried to the trolley or third rail and returns after passing through 
the car motors by means of the track rails and the ground. All feed 







UNDERWRITERS’ REQUIREMENTS 


23 


wires from such generating stations must be protected at the station 
by an approved circuit breaker or similar device which will auto¬ 
matically cut off the current from the feed wires in case of an acci¬ 
dental ground which might allow abnormally large currents to pass 
over the wire. 

Dynamos in manufacturing plants are usually installed in the 
same rooms with the engines which drive them, and such rooms, 
therefore, present possible hazards from the exposure of wiring to 
steam piping and to mechanical injuries from whatever work may 
be done in repairs or alterations of machinery. The most common 
faults in such locations arise from crowding the electrical equipment 
into too small and inaccessible locations from a desire to use space 



Z-W/Pi f. D.C GENERATOR 



2-WIRE GENERATOR 
ON 3-W/RE SYSTEM 

Fig. 20. Diagram Showing Fuse 



) LOAD 

£ A 

j) LOAD 

-U_u- 

2-W/RE GENERATORS ON J-W/RE 
SYSTEM 

6%' 

r 

LOAD • 


L 

MOTOR-GENERATOR BALANCER 


;ion in Various Types of Circuits 


either not available for other uses or not especially planned for 
electrical apparatus. Convenience of operation, cleanliness of sur¬ 
roundings, and a workman-like layout of equipment, all contribute 
to the maintenance of good fire conditions. The requirements for 
overload protection at constant-potential generators are shown in 
the diagram, Fig. 20, where F in each figure represents the fuses or 
circuit breakers required by the Code. These fuses and breakers 
will usually be located on the main switchboard. This leaves 



























24 


UNDERWRITERS’ REQUIREMENTS 


the large main cables from generators to switchboard unprotected 
and for this reason special pains should be taken with their instal¬ 
lation since an accident to them might be very serious. 

Dynamo-Room Wiring. All wiring in dynamo rooms should be 
done in the most substantial manner, and special attention should 
be given to simplicity and orderliness of arrangement. All supports 
for conductors must be very substantial. Fig. 21 shows an excellent 
example of a large mass of wiring well installed. 

All wiring should be readily renewable and accessible. This 
usually requires either open wiring or some form of conduit. A 



Fig. 21. Example of Large Mass of Wiring Well Installed 


fiber conduit is largely used for this work and gives good results. 
The large amount of wire and cable often found in dynamo rooms, 
together with the large currents carried on the conductors, makes 
it necessary especially about switchboards to cover the highly in¬ 
flammable rubber and braided coverings with a tight jacket of some 
non-combustible material, such as asbestos, to reduce the probability 
of fire spreading rapidly over all the exposed wiring. 

Bus bars are usually left bare but there should be as little other 
bare current-carrying metal as possible. 

All conductors should have ample current capacity, and in their 







UNDERWRITERS' REQUIREMENTS 25 

installation special care must be given to securing excellent insula¬ 
tion and reliable supports. Due regard should be given to the 
possibility of injury to conductors by tools, belts, ladders, etc., or 
by any vibration which may cause a gradual wear on wire coverings. 

In general, the requirements of Class C of the Code apply to 
dynamo-room wiring with such special precautions as are dictated 


Fig. 22. Old-Style Type of Wood-Frame Switchboard 

by the large currents to be cared for and the presence of a great 
number of conductors which may have to be placed rather closely 
together. 

Switchboards. The danger of fire at switchboards lies in the 
large number of wires usually concentrated on them, and the use of 
rheostats, fuses, and other appliances which are possible sources of 
fire. The switchboard itself should always be of slate or porcelain. 










26 


UNDERWRITERS’ REQUIREMENTS 


The old wood racks (see Fig. 22) once used are not desirable 
though still permitted if of hard wood well filled. Fig. 23 shows a 
small, well-arranged switchboard. While the front of the switch¬ 
board is the operating side, the rear is the part where trouble is more 
likely to occur. The back of the board should, therefore, be easily 
accessible and neatness of arrangement and reliable supports for all 



Fig. 23.* Approved Type of Small Switchboard 


conductors be imperative. No makeshifts are tolerable on switch¬ 
boards under any conditions. Fig. 24 shows the back of a board. 
Every precaution must be taken to keep water or even moisture 
off switchboards since a short-circuit would be easily started and 
the result would often be highly disastrous. The Code, Rule 3, 
gives in detail some requirements for location and care of switch¬ 
boards, the reasons for which are apparent without further explana¬ 
tion. Fig. 25 shows a large modern switchboard. In general the 
exact arrangement of parts on switchboards is determined by en- 

» Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 











Fig. 24. Back View of Well-Arranged Modern Switchboard 



Fig. 25. Front View of Large Modern Switchboard 



















































28 


UNDERWRITERS’ REQUIREMENTS 


gineering and operating requirements, and the interest, from the 
viewpoint of fire, centers in the protection of conductors and the 
location of board and instruments to reduce the probability of fire 
spreading from the board to the other parts of the building. Order¬ 
liness and cleanliness should be insisted upon. 

Resistance Boxes or Rheostats. These are used in a great 
variety of sizes and designs with dynamos and motors and at switch¬ 
boards. They may be mounted 
either on the machines or switch¬ 
boards or separately, but in every 
case they must be considered as 
“stoves” and liable to become 
“red-hot.” The fact that they do 
not become hot when used under 
normal conditions is not to be 
considered as in any way lessening 
the requirements for protection. 

A burn-out of a rheostat may 
result in a mass of flames from 
the numerous wires, usually con¬ 
tained in or leading to the device, 
and there may be drops or spurts of melted solder or other metal. 
Fig. 26 shows the interior of a small starting rheostat. 

All resistance devices used for starting, regulating, and controlling 
machines are to be classed as “resistances” in the intent of the Rules, 
and so in general are all devices or parts of devices where coils of 
wire, iron grids, or other similar objects are made parts of the circuit. 

Lightning Arresters. The purpose of these devices is to prevent 
lightning, or external high-voltage currents from foreign circuits, 
entering the station and there causing fire or damaging machinery 
and instruments. Lightning cannot be “stopped” but may be 
diverted to the earth. A large variety of lightning arresters are on 
the market but they are all designed to afford a path by which dis¬ 
charges may pass to “ground” instead of into the station apparatus 
and to prevent so far as possible the normal currents on the lines 
from following this path. The operation of many lightning arresters 
is accompanied by sparks which may in severe cases assume the 
size of powerful arcs. The location of these devices away from all 





UNDERWRITERS’ REQUIREMENTS 29 

combustible material is thus imperative. They are often installed 
in separate buildings with other emergency appliances. 

The full discussion of lightning-arrester construction, opera¬ 
tion, and installation is impracticable in this book. Suitable loca¬ 
tion, the running of connections in straight lines with the fewest 


Fig. 27. Electrolytic Lightning Arrester for 7,000 Volts 

possible bends, the use for ground wires of copper wire not smaller 
than No. 6, and especially the securing of permanently good ground 
connections are the chief considerations. Two separate ground con¬ 
nections are often desirable. Fig. 27 shows a type of electrolytic 
lightning arrester for 7,000 volts maximum potential to be used on 
three-phase circuits, grounded or neutral service. 













30 


UNDERWRITERS’ REQUIREMENTS 


Ground Detectors and Tests. Except where circuits are inten¬ 
tionally and permanently grounded, a leak to “ground” may be 
the cause of arcs at any point in the system which may be dangerous. 
By “ground” is to be understood the earth, walls or floors of masonry, 
pipes of any kind, iron beams or floors and the like. It is, there¬ 
fore, required that suitable instruments and circuits be provided to 
indicate either continuously, or by frequent tests, whether there 
are such ground connections which would indicate a failure of some 
insulation. If such a failure is shown by the detector the trouble 
can usually be located and repaired before injury is done. 

Ground detectors may take the form of special instruments on 
the switchboard or of arrangements of switches with lamps or volt- 



Fig. 28. Wiring Diagram for Ground Detector on Any Two-Wire 
Low-Voltage System 


meters which can be used to test the insulation of each side of each 
circuit in turn. The connections should be such that the detector 
cannot be left out of circuit. For the various arrangements of ground 
detectors reference must be made to the standard electrical treatises. 

Attention may be called to the following points which are fre¬ 
quently neglected: 

(1) Lamp receptacles should be keyless and any switches in the 
detector circuits should be so connected that the detector will con¬ 
tinuously give an indication on at least one side of the circuit. 

(2) The wires should always be protected by small fuses where 
they connect to the bus bars. 

(3) If the detector is of a type depending on relative bright¬ 
ness of two lamps, the lamps should be very close together. 

(4) Special care should be taken to secure excellent ground 
connection for the ground wire. 



















UNDERWRITERS’ REQUIREMENTS 


31 


The accompanying diagram, Fig. 28, shows a very good and 
simple detector for any two-wire low-voltage system, and is typical 
of many in use. (The diagram and description are taken from a 
publication of the Associated Mutual Fire Insurance Companies.) 

The lamp for the detector should be of the same candle power and voltage, 
the voltage being about the same as that of the regular lamps in the plant, 
and two lamps should be selected which, when connected in series, burn with 
equal brilliancy. Although somewhat greater sensitiveness can be obtained 
with low-candle-power lamps, such as 8 c. p. for example, it is believed in 
general to be preferable to use lamps of the same candle power as those through¬ 
out the plant, as then a burned-out or broken-detector lamp can be imme¬ 
diately replaced by a good lamp from the regular stock, thus avoiding the 
necessity of keeping on hand a few spare special lamps. 

The detector lamps, being two in series across the proper voltage for 
one lamp, burn only dimly. If, however, a ground occurs on any circuit, 
as at a , the current from the positive bus bar through lamp No. 1 divides on 
reaching b, instead of all going through lamp No. 2, as it did when there was 
no ground. Part now goes down the ground wire and through the ground to 
a, as indicated by the broken line, and thence through the wires to the negative 
bus bar. This reduces the resistance from b to the negative bus bar, and, 
therefore, more current flows through lamp No. 1 than before, while less cur¬ 
rent flows through lamp No. 2. Lamp No. 1 consequently brightens and 
lamp No. 2 dims. If the ground had occurred at c instead of a, lamp No. 2 
would have brightened and lamp No. 1 dimmed. This detector, while not able 
to indicate extremely small leaks, will show any leak that is likely to be dan¬ 
gerous from a fire standpoint. 

Motors. Electric motors are very generally used in manu¬ 
facturing plants of all sorts and their use is still rapidly increasing. 
They are also used very extensively for elevators, cranes, and for 
an infinite variety of uses in all classes of buildings. Recently port¬ 
able motors of small size have come into very general use for drills, 
and other portable tools, vacuum cleaners, washing machines, and 
scores of other purposes where their cleanliness, economy, and 
adaptability to all service recommends them. These portable 
motors, however, do not in general come under the rules prescribed 
for stationary machines, though their use involves similar hazards 
and some others peculiar to themselves. These will be discussed 
later. 

In general the same precautions should be taken with motors 
as with generators. Dry, clean, well-lighted locations should be 
chosen for them. It is a very common fault to install a motor in a 
space too small for it, or in a place where rubbish and dirt are allowed 


32 


UNDERWRITERS’ REQUIREMENTS 


to accumulate. The fact that modern motors are very sturdy ma¬ 
chines, and will continue to work even when neglected and abused, 
encourages carelessness in maintenance which often results in hazard¬ 
ous conditions. Except where unavoidable, motors must never be 
located in the vicinity of combustibles, in wet places, or in very dusty 
or dirty places. When such locations are unavoidable, an “enclosed” 
type of motor should be used, or a special room, Fig. 29, should be 
provided. Many modern types of alternating-current motors have 



Fig. 29.* Approved Housing for Motor in Bad Location 

no exposed live parts and do not have commutators or brushes and 
are, therefore, well suited to dusty or linty places. Fig. 30 shows a 
rough commutator of a motor. It is evident that as the commutator 
revolves, the brush which carries the main motor current will not 
fit well at all points and, therefore, sparking will result. In extreme 
cases this sparking may be dangerous, especially in dusty places or 
where inflammable gases are present. Where motors are used, not 
of enclosed types with brushes, commutators or slip rings, where 
sparking may occur, special enclosures should be provided. The 
installation of motors on ceilings, Fig. 31, or built in as parts of the 
special machines which they drive, often affords both the neatest 

* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 











UNDERWRITERS’ REQUIREMENTS 


33 



BROS# 


COMMUTATOR 


Fig. 30. Rough Motor Commu¬ 
tator Increases Sparking 


and the safest arrangement. Motors operating at voltages over 550 

volts require special precautions in re¬ 
spect to wiring leading to them. Motors 
for less than 550 volts are wired accord¬ 
ing to the general rules for low-poten¬ 
tial systems. 

Wiring to Motors. The supply leads 
to motors must be of a size to carry at 
least 25 per cent more current than that 
for which the motor is rated. This ap¬ 
plies to all types of motors both d. c. and 
a. c. and is required even though the motor 
is not actually being used for its full load. This is called for to 
provide for the currents required 
to start the motor which are 
almost always greater than the 
current needed for continuous 
full load running, and also to 
provide a margin of safety at all 
times. 

Wiring for Direct-Current 
Motors. The Code prescribes 
the safe carrying capacity of 
wires of different sizes, Page 65. 

Thus if a direct-current motor 
requires 40 amperes when work¬ 
ing under full load, the lead wire 
must be of such size as to carry 
50 amperes. A reference to Rule 
16 shows that this will call for a 
No. 5 wire. As No. 5 wire is not 
usually made for electrical pur¬ 
poses it would be necessary to 
use a No. 4 wire. 

The factors which are in¬ 
volved in the determination of 
the proper size of leads to direct- 

. , . i » n Fig. 31.* Approved Ceiling Mounting 

current motors are the following: for Motor 


* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 








































34 


UNDERWRITERS’ REQUIREMENTS 


TABLE I 

Sizes of Conductors in Direct Current* 


Horso- 

Power 

110 Volts 

220 Volts 

500 

Volts 

Concealed 

Open 

Concealed 

Open 

Open 

1 

14 

14 

14 

14 

14 

2 

10 

14 

14 

14 

14 

3 

8 

10 

12 

14 

14 

4 

6 

8 

10 

14 

14 

5 

6 

6 

10 

12 

14 

n 

3 

4 

6 

8 

14 

10 

1 

2 

6 

6 

12 

121 

0 

1 

4 

5 

10 

15 

00 

0 

3 

4 

10 

171 

000 

000 

2 

3 

8 

20 

0000 

000 

1 

2 

8 


c. m. 

c. m. 




25 

250,000 

250,000 

0 

1 

6 

30 

350,000 

250,000 

00 

0 

5 

35 

400,000 

300,000 

000 

000 

4 

40 

500,000 

350,000 

0000 

000 

3 




c. m. 

c. m. 


50 

700,000 

500,000 

250,000 

250,000 

1 


*The question of drop is not taken into consideration in the above table. 


E= volts required by motor 
k = efficiency of motor in per cent 
h.p. = rated horse-power of motor 

Then if 1= current in amperes at full load and at the given 
voltage 

h.p.X746 
EX k 

When I is found, a reference to the Wire Capacity Table, Page 65, 
will show what size of wire is required for current 25 per cent in ex¬ 
cess of I. 

In Table I are given the sizes of conductors in Brown and Sharpe 
gauge required for direct-current motors of respective sizes in h.p. 
and voltages, on the basis of 90 per cent efficiency and 25 per cent 
overload on motor leads. The two columns refer to wiring run 
“concealed” as in conduit or “open” as on cleats. 













UNDERWRITERS’ REQUIREMENTS 


35 


TABLE II 

Sizes of Conductors in Alternating Current* 


SINGLE PHASE 


H. P. 

110 Volts 

220 Volts 

Amps, on 
Mains 

Size of Wire 

Amps, on 
Mains 

Size of Wire 

Concealed 

Open 

Concealed 

Open 

1 

12 

12 

14 

6 

• 14 

14 

2 

23 

8 

10 

11 

12 

14 

3 

33 

5 

6 

16 

10 

12 

4 

44 

4 

5 

22 

8 

10 

5 

53 

2 

3 

26 

6 

8 

71 

• 2 

85 

0 

0 

42 

4 

5 

10 

110 

000 

000 

55 

2 

3 


A 

B 

A 

B 


THREE PHASE, 220 VOLTS 


Horse-Power 

Amperes 
on Mains 

Size of Wire 

Concealed 

Open 

1 

3 

14 

14 

2 

6 

14 

14 

3 

9 

12 

14 

4 

11 

12 

14 

5 

14 

10 

12 

n 

20 

8 

10 

10 

27 

6 

8 

15 

40 

4 

5 

20 

50 

3 

4 

30 

75 

0 

1 

50 

125 

0000 

000 

75 

185 

300,000 

250,000 

100 

250 

500,000 

350,000 

150 

370 

800,000 

600,000 


A 

B 


* Size of conductor in Brown and Sharpe gauge required for alternating-current motors 
of respective sizes in h. p. and voltages, on the basis of 90 per cent efficiency, 50 per cent 
overload, and 85 per cent power factor. 

The question of drop is not taken into consideration in the above table. 

Note. Column “A” gives currents to be used in calculating size of 
mains supplying more than one motor. 

Column “B” gives size of wire for branches and mains supplying one 
motor. 





































36 


UNDERWRITERS’ REQUIREMENTS 


Wiring for Alternating-Current Motors. The calculations for 
proper sizes of wires for alternating-current motors, are somewhat 
more complicated than for direct-current motors. 

Table II gives the wire sizes prescribed by the rules of the 
Department of Electricity of the City of Chicago. Somewhat dif¬ 
ferent values are, however, in use in other places and no universally 
accepted rule has yet been developed. 

Switches and Protective Devices with Motors. Every motor of 
over | horse-power must be protected by fuses or circuit breakers, 
and controlled by a switch; and all of these must be located within 
sight of the motor, and be arranged so as to open and to protect all 
the wires. An automatic circuit breaker which will disconnect all 
the wires of the circuit is considered the equivalent of both the fuses 
and the switch. 



Fig. 32. Wiring Diagram of Motor Circuit Showing Inadequate Fuse Protection 


The chief purpose of the fuses is to protect the motor and espe¬ 
cially the wiring to it from overloads resulting from accidents to the 
motor or from the excessive current which will flow if an attempt is 
made to start the motor when it is for any reason unable to start 
and attain its proper speed. The fuses must, therefore, always be 
of such size as to blow with currents not in excess of the specified 
carrying capacity of the supply wires. Circuit breakers must not 
be set more than 30 per cent above the carrying capacity of the wire 
unless fuses are also used. 

For a. c. motors, each phase (i. e., each main motor circuit 
whether there are one or more) must have a fuse whether circuit 
breakers are used or not. This is because of the necessity of setting 
circuit breakers on such circuits so that they will not open with the 





















UNDERWRITERS’ REQUIREMENTS 


37 


large currents which flow on starting the motor. The breakers, 
therefore, may not protect the wires properly but the fuses act less 
promptly and even if of lower rating will not flow before the motor 
has come up to speed and the currents have been reduced to the 
normal amounts for actual running power. An exception to this rule 
is made in cases where the protective devices are on main switch¬ 
boards or under constant expert supervision. Also for single-phase 
a. c. motors, one fuse and one circuit breaker are allowed, one in each 
of the twm motor supph 
wires. 

The switch at a motor 
is required so that the line 
can be readilydisconnected 
from the motor when the 
latter is not in use or in 
case of accident. On tw T o- 
wire systems, a double¬ 
pole switch is required, on 
three-wire, a triple-pole. 

If a circuit breaker is 
used without fuses it must 
be of a type which will 
protect the motor under 
all circumstances. Thus 
in Fig. 32 the single-coil circuit breaker does not comply with 
the rule, since, if a ground should occur on the main at G and 
another on the motor at G l , the coil C of the circuit breaker would 
be cut out of the circuit and the breaker w T ould fail to operate, for 
no provision is made to open the other line. In this case a fuse of 
proper size should be installed at F or a circuit breaker having a trip 
coil in each side should be substituted for the fuse and breaker shown 
in Fig. 32. If the circuit breaker takes the place of the switch at the 
motor it must be such that one line cannot be opened without open¬ 
ing also all the others. 

Motors of J h.p. or less may be connected to circuits less than 
300 volts in the same way as incandescent lamps, provided the 
proper fuses are used in the branches supplying the power to them. 
Such motors, illustrated by desk-fan motors and all portable motors 



38 


UNDERWRITERS’ REQUIREMENTS 



Fig. 34. Common Type of Motor 
Rheostat 


of small size, do not usually require starting rheostats. Fig. 33 shows 
a small ventilating outfit to be connected to a lamp socket. 

Rheostats with Motors. Rheostats are used with motors for start¬ 
ing them and for controlling their 
speed. They should always be re¬ 
garded as sources of heat and installed 
accordingly. Fig. 34 shows a common 
type of motor-starting rheostat. Such 
rheostats (called motor starters) need 
to be in the circuit foi only the very 
short time usually required to bring 
the motor up to speed and, therefore, 
they are usually so designed to carry 
the necessary current for only this 
brief period. Such motor starters, 
if left with current passing through their coils or grids for longer 
periods, will become very hot and may burn out. The manner in 
which a simple motor starter is connected is shown in Fig. 35 when A 
represents the armature of the motor, M 
the field coil or electromagnet of the motor 
and R the resistance coils in the starter 
rheostat box. The required fuses and 
switch are shown at FF and S. In this 
design the starter, fuses, and switch are 
all mounted on a single slate base. To 
start the motor, the switch S is first 
closed and the handle II is slowly moved 
from 0 to the position shown in the dia¬ 
gram. It will be seen that the resistance 
R during the process of starting serves to 
limit the current supplied to the motor ar¬ 
mature, which would otherwise, until the 
motor came up to speed, be excessively 
large. When the handle has reached the 
position shown, the resistance is all cut 
out of the circuit. A rheostat of this sort cannot be used safely 
except to start a motor of the proper size for it. The resistances will 
get excessively hot if II is allowed to remain at any intermediate point. 



Fig. 35. Starting Rheostat Wiring 
Diagram 













UNDERWRITERS’ REQUIREMENTS 


39 


Motor starters for d. c. motors must, therefore, be made so that 
a spring or other device will return the lever to the “off” position in 
case the operator attempts to leave the starter with current passing 
through its resistance coils. This is because these coils are not 
designed for this continuous duty. When, however, the lever has 
been moved clear across to itsTinal position, it is held there by a small 
electromagnet (V in Fig. 35) but in this position the connections 
have been shifted so that the resistance coils are no longer in the 
circuit. If now it should happen that the supply of current should 
fail, the electromagnet (called the no-voltage release) will release the 
lever which will fly back to the original “off” 
position. Then when the supply is re-estab¬ 
lished the motor will not be injured by start¬ 
ing too suddenly, or by the severe arcing at 
the motor commutator. 

Since a. c. motors usually have no com¬ 
mutators and are less liable to injury from 
sudden starting, a no-voltage release is not 
required in a. c. starters. 

A somewhat different device called an 
autostarter, potential starter , or compensator, is 
often used for starting a. c. motors. It is a 
form of transformer which allows only a por¬ 
tion of the line voltages to be applied to the 
motor until it is well started. Autostarters 
are to be regarded as sources of heat, and 
much the same precautions are necessary as 
with ordinary starters containing wire coils or Fig. 36. interior view of 

. . Compensator 

iron grids. If they contain oil-immersed 

switches or coils, the oil adds a further hazard on becoming overheat¬ 
ed or spattered about. Fig. 36 shows the interior of a compensator. 

In general, motor starters, either d. c. or a. c., involve both the 
hazards of switches and those of possibly overheated coils of wire or 
other parts, and must be treated accordingly. 

Another class of rheostats includes those which are intended for 
regulating the speed of motors and generators. They are necessarily 
used continuously whenever the machine is running and, there¬ 
fore, must be proportioned so that their coils or other resistances 







40 


UNDERWRITERS 7 REQUIREMENTS 


will not become overheated, even when the necessary current traverses 
them for hours at a time. Fig. 37 shows a large rheostat of this type. 
Among these rheostats are classed field rheostats for dynamos, con¬ 
trollers for motors (as distinguished from motor starters), theater 
lamp dimmers, and, in fact, all rheostats or resistances which are 
required to be continually in the circuit. Some motor starters are 
designed to serve also as controllers and may have two sets of re¬ 
sistances, one for starting only and the other for continuous duty 
in varying the speed of the motor. These are especially useful in 
connection with machine tools and similar apparatus. 

Special forms of rheostats having their resistances so enclosed 
that a burn-out will not cause sparks, are required in dusty or linty 



Fig. 37. Closed and Open Views of Large Rheostat for Continuous Service 


places, such as flour mills, various textile mills, etc. If such spe¬ 
cially protected starters are not supplied, the starting apparatus 
should be in special rooms where the dust or lint can be kept out. 

The dangers to be guarded against in all rheostats of every type 
arise from the fact that these appliances must from their very nature 
be sources of heat. By liberal design and good ventilation the 
temperatures can be kept low; but all rheostats should be installed 
with the idea that they may become overheated by failure or mis¬ 
use, Fig. 38. The large amount of insulated wire often used, and 
the oil in which parts are sometimes immersed, may furnish a con¬ 
siderable amount of fuel, and in severe burn-outs, flames may result 
and molten metal be ejected. A large amount of smoke usually 
accompanies a burn-out of a rheostat. 










UNDERWRITERS’ REQUIREMENT! 


41 


Storage Batteries. Batteries of large size are now in common 
use. These are practically always storage batteries, that is, bat¬ 
teries which are recharged by having current from a generator 



Fig. 38. Example of Badly Installed Rheostat and Switch 

supplied to them from time to time. What are called primary 
batteries, that is cells in which there is a chemical action which sets 
up a current without any machine being used to charge and recharge 










42 


UNDERWRITERS’ REQUIREMENTS 



them, are sometimes of large size, and develop fairly high voltages. 
They are, however, used chiefly for telegraphs and other signaling 
work and need not often come under the supervision of underwriters 
or fire-protection engineers. 

Storage cells, however, where they are capable of developing 
the same voltage (100 to 600 volts not uncommon) require the same 
general precautions as dynamos or motors since they produce like 
amounts of energy. Furthermore, it is a property of such batteries 

that in case of a short- 
circuit they can for a 
short time supply very 
large currents, far larger* 
than they are normally 
called upon to supply. 
Thus they may be term¬ 
ed reservoirs of energy, 
capable of producing 
trouble if their output is 
not properly controlled. 

Storage battery rooms 
should be thoroughly 
ventilated. The action 
of the current in charging 
the battery, liberates at 
times large quantities of 
hydrogen and oxygen, 
and if these should ac¬ 
cumulate in the right 

Fig. 39. General Electric Arc Lighting DrODOl’tions thev WOllld 

Transformer with Series Rectifier piupui uun&, unry WUU 1 U 

form an explosive mix¬ 
ture which might be exploded by any accidental spark. 

The water and acid used about storage batteries make it neces¬ 
sary to provide especially good insulation. The battery jars are 
best mounted on glass strips set on special porcelain insulators. The 
usual precautions for rooms where acid fumes exist should be taken 
in battery room wiring, and metal parts must not be such as to be 
affected by corrosion, since a decrease of the cross-section area of 
any current-carrying part may ultimately reduce the metal to a 












UNDERWRITERS’ REQUIREMENTS 


43 


degree that will cause overheating or, in case of an actual break, a 
dangerous arc. 

Transformers. In central or substations or in large power 
rooms of isolated plants or factories, the chief point in the installa¬ 
tion of transformers is the provision for preventing injury, the smoke 
resulting from burning out of the coils or, in the case of oil-filled 
cases, from the boiling over of the oil. These effects are, of course, 
produced onty from some accident to the apparatus or circuits, 
but transformer fires are peculiarly difficult to fight, and the oil and 
the insulations on the windings produce, at high temperature, large 
volumes of smoke which may damage goods in other parts of the 
building than the transformer room itself, or which might be mis¬ 
taken for a fire and result in water being thrown into the building 
entailing a heavy “water loss.” Transformers should always be 
located in clean, dry places with ample space about them. Fig. 39 
shows a General Electric constant-current arc-lighting transformer 
with series rectifier outfit for 4-ampere 50-light system. 

OUTSIDE WORK 

Defects or failure of electric light or power circuits outside of 
buildings, as on poles or over the roofs or walls of buildings, may 
become the cause of fire by setting up abnormal conditions in ap¬ 
pliances connected to them, by setting fire, either by overheating or 
arcing, to buildings on which they are supported, or especially by 
crosses. By a cross is meant an accidental touching by wires of one 
circuit or line with those of another line. By such accidental con¬ 
tacts lines may become charged with a higher voltage than they or 
their appliances are suited for. Thus a cross between a 2,200-volt 
arc-light wire and a 110-volt house-lighting circuit may cause current 
from the 2,200-volt circuit to pass into one or more houses where 
it will perhaps blow fuses with an explosive action, or cause very 
dangerous arcs as it passes to the ground through some defective 
insulation, Fig. 40. Similarly it is very dangerous for an ordinary 
lighting circuit to become crossed with a telephone wire, since the 
insulation and carrying capacity of telephone wiring is not such as to 
resist the effects which might be produced. For reasons such as these, 
outside wiring has a definite relation to fire hazard even in buildings. 


44 


UNDERWRITERS’ REQUIREMENTS 


Wiring. In very many cities and towns insufficient attention 
was paid to outside wiring until the streets and alleys became crowded 
with circuits of all sorts, each put up as was cheapest or most 
convenient at the time it was constructed. Where some of these cir¬ 
cuits are of high power and high voltage exceedingly dangerous 



Fig. 40. Result of Cross Between 2,200-Volt Lighting Circuit and 110-Volt House Circuit 


conditions exist, and the hazard may be extended over large areas 
remote from the place where actual failure occurs, since the resulting 
accidental currents may pass over any of the lines involved to build¬ 
ings at considerable distances. The efforts of fire-protection engineers 
and insurance interests should be directed toward the correction 
of faulty outside wiring conditions, which if neglected are bound to 





UNDERWRITERS’ REQUIREMENTS 


45 


become progressively worse and constitute a serious menace to lives 
and property. 

Where an insured property is clearly jeopardized by outdoor 
wiring, excellent electrical conditions indoors can at best afford only 
a partial safeguard, and fire-protection engineering should apply 
itself to remove the defects out-of-doors as well as in. 

Underground System. The placing of electric wires underground, 
especially where low voltage and signal circuits are carried in separate 
conduit systems, affords the most thorough solution of the hazards 
of town and city wiring. Underground wires are, of course, less liable 
to accidents of weather and storm, and are, therefore, preferred by 
operating companies although the expense of putting wires under¬ 
ground often prevents or postpones a change. 

Legislation prescribing a gradual change, a 
certain amount of wiring being put under¬ 
ground each year, has often produced results 
without undue hardship to electric com¬ 
panies. However, there are certain well, de¬ 
fined precautions and standard methods for 
outside work which can reduce hazards to a 

great extent. _ . Fig. 41. Petticoat Insulator 

Standard Practice. Line wires should 
have either rubber insulating or weatherproof covering. The 
latter consists of three tight cotton braids, each thoroughly im¬ 
pregnated with a waterproof compound. All tie wires, as at 
insulators, must have an insulation equal to that of the wires 
they confine. The insulation of outside wires on poles is re¬ 
quired as an additional protection, but is not in any way de¬ 
pended on for insulation at the supports. The use of bare wires 
would greatly increase the probability of crosses in cases of break¬ 
age of wires. 

Standard practice is to use glass or porcelain insulators at least 
1 foot apart on pole cross-arms and to ground all metallic sheaths 
of cables. Fig. 41 shows a cross-section of one form of porcelain 
insulator. The “petticoats” will nearly always have a dry space 
underneath their lower edges, and even if not dry, the length of the 
path offered to the current escaping over the wet surface is so great 
that the leakage is small. 







46 


UNDERWRITERS’ REQUIREMENTS 


All joint wires should be soldered and carefully insulated. The 
joint should be made mechanically and electrically secure before 
the solder is applied. Good joints are requisite to lessen the chance 
of wires falling as well as to prevent arcs on the wires themselves. 

Proximity of Wires to Electric Light and Power Lines. Probably 
the most common and also the most dangerous fault in outside work 



Fig. 42. Presence of Overhead Wires Hinders Firemen 


is the running of telephone and other signal wires too close to electric 
light and power lines. Wires of this sort should never be on the 
same cross-arms and it is much the best practice wherever possible 
to keep telephone wires on poles on one side of a street, and light 
and power wires on the other, giving special attention to the neces¬ 
sary crossings, if any. In all outside pole work in towns due regard 
should be given to the fact that lines consisting of many wires are 
a serious hindrance to firemen in attacking a fire in the adjacent 











Fig. 43.* Well-Installed Roof Wiring 

Outside Wires on Buildings. Where outside lines are supported 
on buildings, they should be at least 7 feet above the highest point 
of flat roofs and at least 1 foot above the ridge of pitch roofs over 


Fig. 44.* Approved Form of Roof Structure for Safe Installation 

which they pass and the roof structures should be of the most sub¬ 
stantial description. Figs. 43 and 44 show two forms of roof struc¬ 
ture which hold the wires high enough so that they cannot sag and 

* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 


UNDERWRITERS’ REQUIREMENTS 47 


buildings, Fig. 42. The disposition of the lines should be such as 
to reduce this as far as practicable; but placing wires underground is 
the only complete solution of this problem. 












48 


UNDERWRITERS’ REQUIREMENTS 


touch the roof and so that they are not liable to be touched or dis¬ 
turbed by persons walking on the roofs. Where outside wires are 
brought from pole lines into buildings special precaution must be 
taken. Such wires are spoken of as service wires or services. 

The portion of such wires from the service switch in the building 
and the first outside support must be rubber insulated. Every pre¬ 
caution must be taken to keep service wires free from contact with 
cornices, awning-frames, shutters and the like, under all conditions. 

Usually, for low-volt age cir¬ 
cuits, it is best to put the 
wires in metal conduit, as 
shown in Fig. 45. This draw¬ 
ing also shows the drip loops 
which should always be pro¬ 
vided, the insulators on brack¬ 
ets secured to the wall and the 
special pipe-cap on the top of 
the conduit to keep rain out 
of the pipe. Where conduit 
is not used, porcelain bushings 
may be used, slanting upward 
through the wall toward the 
inside. 

In general it is better to 
have service wires enter 
through the basement rather 
than through an upper story 
or attic. Where wires are 
carried along side walls out¬ 
side of buildings, they should 
be supported on glass or por¬ 
celain insulators, not more than 15 feet apart. Where not exposed to 
weather, porcelain knobs may be used if not more than 4J feet apart. 

Trolley Wires. Trolley wires must be of ample size for mechanical 
strength (No. OB. & S. gauge copper or No. 4 B. & S. gauge silicon 
bronze). Protection against crosses must be ample and street rail¬ 
way trolleys and feeder cables must be capable of being disconnected 
at the power station or of being divided into sections so that in case 































UNDERWRITERS’ REQUIREMENTS 


49 


of fire on the railway route, the current may be cut off from the 
particular section and not interfere with the work of the firemen. 

Electrolysis. Whenever an electric current passes between a 
pipe or wire underground into damp earth an electro-chemical effect 
is produced which may, under certain conditions, produce a disin¬ 
tegrating effect on the metal pipe or wire and eventually destroy it. 
Whenever an electric current passes through a conducting liquid 
which is not a chemical element, the liquid is decomposed. The 
salts and acids in the liquid are thus decomposed and metal plates 
by which the current is led into and out of the liquid are also affected. 
This process is called electrolysis, that is, breaking up of chemical 
composites by electric currents. 

It is at once evident that water and gas pipes of iron buried in 
moist earth provide the necessary combination for electrolytic effects 
and that pipes in areas traversed by currents through the earth may 



Fig. 46. Diagram of Electrolytic Effects of Street Railway Return Currents 

be injured by such chemical disintegration. In cities and towns 
where there are large electric currents which use the earth as a 
“return path,” these effects, if improper conditions are allowed to 
exist, may become very serious and have a bearing on the fire hazard 
chiefly as they may jeopardize the water mains and supply pipes to 
an extent sufficient to render the fire-fighting facilities unreliable in 
emergencies. Since the direction of alternating currents reverses 
many times each second, the chemical effects which they can produce 
are also reversed and, therefore, in general alternating currents in the 
earth are not liable to produce dangerous electrolysis. There are, 
however, direct currents which usually employ ground returns and 
by far the commonest are the currents of the usual overhead trolley 
street railways. It is to these, therefore, that the following explana¬ 
tion applies. Fig. 46 illustrates the way in which the current in a 
trolley circuit returns to the generator. Such circuits in this country 



























50 


UNDERWRITERS’ REQUIREMENTS 


are almost invariably direct current at 500 to 650 volts and the 
positive side of the dynamo is regularly connected to the overhead 
wire. The current thus passes out over the trolley, through such 
cars as are in operation and back to the negative side of the dynamo, 
by the rails, or through pipes, moist earth or other conducting sub¬ 
stances underground. The negative side of the generator is, of 
course, connected to the rails and ground. The return current divides 
according to the conductivity of the different paths afforded it and 
the actual distribution of currents in the earth may be very com¬ 
plicated and is subject to great variations from point to point. 
Careful “electrolytic surveys” are necessary in order to discover 
fully the actual current arrangements existing underground in any 
town or city. 

In general, however, it is important to notice that wherever 
the current leaves a pipe for the earth, as at B in Fig. 46, the iron of 
the pipe is carried away into the earth, just as in a silver plating 
battery, silver is carried away from the silver plate toward the 
article which is being plated. It is this action which is meant by 
electrolysis as the term is usually employed with reference to under¬ 
ground pipes and earth currents and the result, as has been stated, 
may be a destruction of the pipe at one or at many points. 

For the diminution of such electrolytic corrosion every means 
should be taken to afford a low-resistance path for the return circuit. 
This is also of advantage from the viewpoint of economical opera¬ 
tion since it takes power to drive the current through a high-resistance 
return path. Large copper conductors are often laid between the 
rails and parallel to them, frequent heavy cross wires being used to 
connect this copper cable to the rails. Good bonds between rails 
either of low-resistance tie plates, or still better of electric welds 
between rail ends, are of great value. 

The general arrangement and size of return wires and rail 
should be such that the difference of potential in volts between the 
grounded terminal of the generator and any point on the return cir¬ 
cuit will not be more than 25 volts. 

Where pipes or other underground metal work are found to be 
electrically positive to the rails or surrounding earth, that is, so 
that current tends to flow from the pipes to the rails or earth , special 
conductors should be provided to connect the pipes to the rails to 


UNDERWRITERS’ REQUIREMENTS 


51 


carry such currents and thus prevent, as far as possible, the effects of 
electrolysis. Very elaborate investigations of electrolysis as affecting 
water mains, gas pipes and other underground metal have been made 
in many cities and the co-operation of city authorities, water com¬ 
panies, and electric power and railway companies, has in many places 
resulted in greatly lessening, if not in entirely removing, the dangers 
from this cause. Surveys should be repeated occasionally since 
new electric circuits or new pipe lines may create an altered condi¬ 
tion in underground distribution of currents. 

High Tension Lines. In recent years there has, in most parts 
of the country, been a very great increase in the number of high- 
tension power-transmission lines and the voltages have also become 
much higher. Overhead lines of this class present certain problems 



Fig. 47.* Crossover Arrangement for High-Tension Circuits 


requiring very careful attention. The Code states the following 
causes of fire which may come from high-voltage (over 5,000) lines: 

Accidental crosses between such lines and low-potential lines may allow 
the high-voltage current to enter buildings over a large section of adjoining 
country. Moreover, such high-voltage lines, if carried close to buildings, 
hamper the work of firemen in case of fire in the building. The object of the 
rules is . so to direct this class of construction that no increase in fire hazard 
will result, while at the same time care has been taken to avoid restrictions 
which would unreasonably impede progress in electrical development. 

It is fully understood that it is impossible to frame rules which will 
cover all conceivable cases that may arise in construction work of such an 
extended and varied nature, and it is advised that the Inspection Depart¬ 
ment having jurisdiction be freely consulted as to any modification of the 
rules in particular cases. 

* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 











52 


UNDERWRITERS’ REQUIREMENTS 


The very best way to guard against accidental crosses between 
high-tension lines and other circuits is to have them follow different 

routes. This can often be ac¬ 
complished by mutual agree¬ 
ment of the parties interested 
even when a change in one 
of the routes will be neces¬ 
sitated. 

High-tension lines should 
not approach other pole lines 
nearer than a distance equal 
to the height of the taller 
pole and such lines should 
not be on the same poles 
with any other lines except 
such signal lines as may be 
used by the company opera¬ 
ting the high-tension system. 

Where such lines must necessarily be carried nearer to other pole 
lines than is specified above, or where they must necessarily be carried 
on the same poles with other wires, extra precautions to reduce the 
liability of a breakdown to a minimum must be taken, such as the 
use of wires of ample mechani¬ 
cal strength, widely spaced 
cross-arms, short spans, double 
or extra heavy cross-arms, extra 
heavy pins, insulators, and poles 
thoroughly supported. If car¬ 
ried on the same poles with other 
wires, the high-pressure wires 
should be carried at least 3 
feet above the other wires, but 
this arrangement should never 
be adopted unless it is impos¬ 
sible to do otherwise. Where 
such lines cross other lines, the poles of both lines must be of heavy 
and substantial construction. 

The Code contains quite detailed specifications for the safe 

* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 



Fig. 49.* Use of Screen on High-Tension 
Crossover 



Fig. 48.* Joint-Pole Crossing with Mechanical 
Guards 






















UNDERWRITERS’ REQUIREMENTS 


53 


construction of crossovers and other details of high voltage lines 
which should be carefully studied and followed. 

Fig. 47 shows one arrangement for a crossover. A joint-pole 
crossing may sometimes be used and Fig. 48 shows such a crossing 
with mechanical guards and wires on the upper or high-tension line 
cross-arms. Sometimes a screen either supported on high-tension 
insulators or grounded may be suspended between the lines at the 
crossover as illustrated in Fig. 49. When necessary to carry high- 
tension lines near buildings they should be at such height and dis¬ 
tance from the building as not to interfere with firemen in event of a 
fire. Such interference might arise either from the difficulty of plac¬ 
ing ladders or from the danger 
of shocks to firemen holding 
hose nozzles, streams from which 
might strike the high-tension 
lines. 

Mounting of Transformers. 

Oil-cooled transformers should 
not, in general, be installed in 
buildings, and an outside location 
is always preferable; first, be¬ 
cause it keeps the high-voltage 
primary wires entirely out of the 
building; and second, because of 
the possible injury from smoke 
and oil in case the transformer 
burns out or is overloaded. Figs, 
installed on a pole, on the outside wall of a mill and in a special fire¬ 
proof vault, respectively. 

Grounding of Circuits. One of two courses should always be 
followed with regard to any electrical connection between circuits 
or apparatus and the earth; either arrange to have no such connection 
at all, and secure good insulation between live parts of circuits and 
the earth, or provide a good earth connection of sufficient capacity 
well installed to care for any current liable to pass to earth, either 
regularly or in case of accident. 

In general it should be remembered, that if an entire electric 
circuit including the generator, lines, and all connected apparatus 



Fig. 50. Approved Pole Installation of 
Transformer 


50, 51, and 52 show transformers 











* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 


54 UNDERWRITERS’ REQUIREMENTS 


and devices are thoroughly well insulated from the ground, one 
accidental ground will produce no effect since there is no return 
path from the ground to another point on the system. However, 
we cannot be sure that an installation will be kept free from grounds 
even if originally so installed, since wear, deterioration of insulating 
materials, breaking of parts of devices, or the effects of dirt and 

damp may bring on a “ground” 
which will not be discovered 
until a second ground connection 
is established which permits cur¬ 
rent to flow with consequent 
heating and arcing. Under such 
conditions, the fault is liable to 
become rapidly worse. Not the 
least important thing about such 
failures of electrical circuits, is 
the danger of injury to persons 
who may become a part of the 
ground circuit. Thus a mechanic 
working on a line shaft in a mill, 
by touching simultaneously a live 
part of the circuit or a poorly 
insulated live wire and the shaft¬ 
ing, may make his body part of 
a path for the current to ground. 
While 110 volts or 220 volts 
either a. c. or d. c. are rarely 
enough to kill a person, painful 
accidents may be caused directly 
or indirectly by shock, and voltages of 440, 500, or higher may 
readily be the cause of death if the conditions are such as to permit 
any but very small currents to pass through the body. While, of 
course, liability of injury to persons is not a “fire hazard,” still 
good construction should provide (both in original installation 
and in upkeep) all reasonable protection, to persons and property. 

In direct-current three-wire systems of electrical distribution, 
the middle or neutral wire is regularly grounded at the central station. 
In systems where the cables are underground, the neutral must also 


Fig. 51.* Approved Transformer Installation 
on Outside of Building 








UNDERWRITERS’ REQUIREMENTS 


55 


be grounded at each distributing box, through the box; and in over¬ 
head systems the neutral should be grounded every 500 feet. The 
neutral in such systems is supposed, normally, to carry either no 
current, or such small amount of current as may result from a dif¬ 
ferent amount of power being temporarily taken from the two outside 
wires. If in any system the neutral is grounded at all, it should be 
done thoroughly so as to prevent the current escaping to ground 
where the connections may be so poor as to cause unsafe heating. 

Two-wire direct-current systems are not to be grounded at all. 
Suppose in Fig. 53 some current-carrying part of the dynamo D is 
grounded, that is, in electrical 
connection with the earth, as 
through the bolt and the damp 
wood base of the machine. If 
this be all, no current will flow 
so long as there is no other 
ground connection to any other 
part of the circuit. But suppose 
that somewhere in the building a 
wire touches a gas pipe as at B 
and the insulation on the wire at 
B is worn by vibration or is de¬ 
fective for any reason. The gas 
pipe is, of course, connected to .the 
earth and the current then has a 
path through the pipe, earth, 
bolt, base, and frame of the ma¬ 
chine. The whole voltage of the dynamo may, therefore, drive current 
through this path, causing an arc to form at B between the wire 
and the pipe which will burn a hole in the pipe and set fire to the 
escaping gas. Evidently the greatest pains should be taken to prevent 
wires from coming in contact with grounded piping or other metal. 

Alternating-current systems almost invariably include trans¬ 
formers in which the higher line voltage is “stepped down” to the 
voltage required for lamps and motors. Common line voltages are 
1,100, 2,200, and 3,300 volts, while the secondary voltages may be 
100, 110, 220, 440 or 550. With such systems as are commonly used 
for distributing light and power, it is preferable to ground the sec- 

* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 



Fig. 52.* Transformer Installation in 
Fireproof Vault 






56 


UNDERWRITERS’ REQUIREMENTS 


ondary or low-voltage side. The ground connection may be made 
either at the transformers or at individual service entrances. At 
transformers the connection is made at the neutral or middle point 
of the secondary winding or coil. With three-wire a. c. distributing 
systems the neutral wire itself is grounded. Sometimes, when a 
neutral point is not accessible, one side of the secondary circuit may 
be grounded, provided it will not establish, between the ungrounded 
side and the earth, a difference of potential of over 250 volts. A 
greater potential difference than this would cause all insulators on 
the system to be subjected to undue strain and perhaps cause trouble. 

It should be remembered 
that a poorly made earth con¬ 
nection may be worse than none. 
The rules, therefore, prescribe in 
detail how to secure good con¬ 
nections. The wire should be of 
large size and should be run in 
as nearly a straight line as pos¬ 
sible, avoiding kinks, coils, and 
sharp bends which are objection¬ 
able since they impede the flow 
of alternating-current or light¬ 
ning discharge. 

Individual transformers and 
building services may be ground¬ 
ed to water pipes by carrying 
the ground wire into the base¬ 
ment and connecting it to the 
street side of meters, main cocks, etc., so that any resistance which 
these might offer in the ground path might be avoided. 

The underwriters’ rules give the following directions for making 
ground connections: 

In connecting a ground wire to a piping system, the wire should be 
sweat into a lug attached to an approved clamp, and the clamp firmly bolted 
to the water pipe after all rust and scale have been removed; or be soldered 
into a brass plug and the plug forcibly screwed into a pipe-fitting, or where 
the pipes are cast iron, into a hole tapped into the pipe itself. For large stations, 
where connecting to underground pipes with bell and spigot joints, it is well 
to connect to several lengths, as the pipe joints may be of rather high resistance. 



Fig. 53. Faulty Installation of Two-Wire 
Direct-Current Dynamo 





































UNDERWRITERS’ REQUIREMENTS 


57 


Where ground plates are used a No. 16 Stubbs’ gauge copper plate, about 
3X6 feet in size, with about 2 feet of crushed coke or charcoal, of pea size, 
both under and over it, would make a ground of sufficient capacity for a moder¬ 
ate-sized station, and would probably answer for the ordinary substation or 
bank of transformers. For a large central station, a plate with considerably 
more area might be necessary, depending upon the other underground con¬ 
nections available. The ground wire should be riveted to the plate in a num¬ 
ber of places, and soldered for its whole length. Perhaps even better than a 
copper plate is a cast-iron plate with projecting forks, the idea of the fork 
being to distribute the connection to the ground over a fairly broad area, and 
to give a large surface contact. The ground wire can probably best be con¬ 
nected to such a cast-iron plate by soldering it into brass plugs screwed into 
holes tapped in the plate. In all cases, the joint between the plate and the 
ground wire should be thoroughly protected against corrosion by painting it 
with waterproof paint or some equivalent. 

In the past few years, there has been much discussion of this 
question of grounding, but the general tendency is no doubt more 
and more in favor of grounding all circuits where so doing will 
protect life and will not introduce extremely hazardous conditions as 
regards fire. The following statement of arguments for and against 
grounding is taken from the explanatory notes of the Associated 
Factory Mutual Fire Insurance Companies: 

If the primary and secondary coils of a transformer come into contact 
electrically, the high-voltage primary current may flow to the secondary 
system. If this should happen, the life of any one handling any part of the 
secondary system would be endangered, and fires would probably be started 
by arcs caused by breaking down of the insulation of the wires or fittings on 
the secondary system. If, however, the secondary coil is grounded, a break¬ 
down in the transformer cannot cause a dangerous difference of potential be¬ 
tween the secondary system and the ground, and only with certain unusual 
combinations of contacts between the primary and secondary wires outside 
of the transformers will this protection fail to prevent the voltage of the sec¬ 
ondary system from being raised above its normal limit. In order to secure 
the full benefit of the ground connection, reliable primary fuses of proper 
carrying capacity must be provided. 

The middle of the secondary coil is the proper point to ground, as there 
is then only half the normal secondary voltage between either side and the 
ground, thus reducing the liability of a breakdown of insulation and also 
materially lessening the danger of fire if a breakdown does occur. 

There is an objection to grounding the secondary on the other hand, 
for when this is done, the first breakdown of insulation may mean a short- 
circuit and a possible fire. With a system free from grounds, a breakdown 
must exist on each side of the system to cause a short-circuit, and with proper 
ground detectors the first can generally be discovered and remedied before 
the second occurs. 

Grounding is, therefore, a choice of evils, but in many cases it is believed 
to be a lesser one than to risk getting the primary current on the secondary 


58 


UNDERWRITERS’ REQUIREMENTS 1 


system. This is especially true where the primary voltage is high, say 3,500 
or over. For this reason it is advised that all transformers be so designed and 
connected that the middle point of the secondary coil can be reached if, at 
any future time, it should be desired to ground it. 

After the transformer secondary has been properly grounded a test 
should be made, especially if the transformer is some distance from the build¬ 
ing supplied, in order to determine if the protection expected from the ground 
connection at the transformer is really effective inside the building in ques¬ 
tion, and if not the connection should be extended to accomplish the desired 



Fig. 54. Effect of Short-Circuit from Lighting 
Wires to Bell Circuit 

result. Cases have been known where the effectiveness of a ground connection 
has been limited to a comparatively small area, due to the exact conditions 
of the earth in the neighborhood of the ground plate and between it and the 
point where the protection due to the grounding was desired. The entire 
ground connection should be carefully examined at least once a year. 

INSIDE WORK 

Under this heading are included the rules for wiring and ap¬ 
pliances for light, heat, and power distribution and use. These cover 


















UNDERWRITERS’ REQUIREMENTS 


59 


the most general cases of electrical installations as they relate to fire 
prevention and protection in buildings of all classes. The installa¬ 
tion of wires and apparatus for signaling systems, such as electric 
bells (battery current bells), telegraphs, telephones, fire alarms, and 
the like, is not covered in the general rules for “Inside Work” since 
they usually present no hazard in themselves but only as they may 
become dangerous because of their liability to become crossed with 
light, heat, or power wires either outside or inside of buildings, 
Fig. 54. 

WIRING SYSTEMS 

The present approved methods of electrical work inside build¬ 
ings in this country have been developed through many years of 
experience, beginning with the first applications of electricity for 
lighting buildings and gradually changing as the possible dangers, 
became more generally recognized, and as improved means of guard¬ 
ing against them were devised. In this development the efforts of 
insurance and municipal authorities have been supplemented by 
an immense activity on the part of inventors and manufacturers in 
supplying new devices and materials. The net result has been on 
the one hand an elaboration of rules and an approach to a few stand¬ 
ard systems of construction and on the other hand the production of 
an almost endless variety of materials available for electrical pur¬ 
poses. Methods and materials which at first seemed adequate have 
become obsolete after a few years’ use. At present, however, few 
important changes appear to be in progress but in many minor 
details the development is still going on. 

At first, electric wires were laid as seemed most convenient in 
floors, partitions, and over walls and ceilings, either in channels cut 
for them, in wood casings, or supported on wood cleats with 
almost no regard to protecting the wires from injury, or the 
adjacent combustible materials from being ignited by overheated 
wires or by arcs. Today, however, these earlier crude methods 
are wholly abandoned and it is generally conceded that the best 
protection against electrical fires lies in the adoption of the most 
approved methods even when the first cost of an installation is 
increased to some extent. Fig. 55 shows some defective wire joints 
as found in actual use. 

In buildings of the better class, the electric installation is care- 


60 


UNDERWRITERS’ REQUIREMENTS 


fully considered in the plans, and provision is made for its safety 
as well as for its efficiency and economy. However, in cheaper 
buildings, and very often in small stores, apartments, and residences, 
the electrical work is left to be arranged as best it may and to be 
installed by careless workmen without expert supervision. In 



Fig. 55. Typical Defective Joints in Electric Installation 


electrical matters as in most other affairs, cheap work is generally 
poor work, and deviation from the methods shown by experience to 
be reliable are usually prompted by a desire to save money at the 
expense of safety and permanence. 

The method of bringing supply or “service” wires into buildings 
naturally demands first attention. Where-street mains are under¬ 
ground the lines should, wherever possible, enter through the base- 





UNDERWRITERS’ REQUIREMENTS 


61 


ment or cellar walls, the lead-covered cables being carried through 
the foundations through tubes tightly sealed. 

Where mains are overhead, the supply wires may enter either 
through the basement, being carried down the outside of the wall 
in iron conduit, or through some upper portion of the wall, though 
the former entrance is almost always preferable. Where transformers 
are used they may be on poles near the building or mounted on the 
outside wall in a substantial manner. An entrance through a roof, 
near a cornice or into little used or inaccessible attics or lofts, should 
be avoided. Where wires or cables pass through the outside walls 
there must be either iron conduit or insulating bushings sloping 
upward toward the inside, and the wires outside must have drip 
loops which will prevent moisture following along them into the wall. 
The fastenings of the wires to the 
building must be most substantial 
and good insulators must be provided 
for the supply wires. Fig. 56 shows 
the method where bushings are em¬ 
ployed. If the entrance is made 
through conduit, the inner end of the 
conduit should always be extended to 
the service fuses. At the nearest 

.. . Fig. 56.* Method of Using Bushings 

accessible place in the building must 

be placed what are called the service fuses and the service switch. 
The switch is usually of the knife-blade pattern and must be such 
as to cut off all the wires. Single-pole switches must never be used 
as service switches. The purpose of service switches is to provide 
means for cutting off current for repairs or in case of fire or other 
accident. The service fuses should be placed between the service 
switch and the mains to the outside of the building. Their purpose 
is to protect all the wires inside the building from overloads, and 
they should be such as to melt or “blow” with current not much 
in excess of the normal full current likely to be taken by the entire 
installation which the service supplies. 

Service switches and fuses take many forms, from a pair of 
small “plug fuses” and knife switch on a porcelain base to quite 
elaborate switchboards which carry both these and other sub-fuses 
for the main circuits within the building. The same principle, how- 

* Courtesy Inspection Department, Associated Factory Mutual Fire. Insurance Companies, Boston, Mass. 





62 


UNDERWRITERS’ REQUIREMENTS 



ever, applies to all; all wires must be protected, the service entrance 
must be accessible and fuses must be of proper capacity. From the 
service switch the lines usually extend through meters to distribution 
centers which are panels from which the several sub-circuits branch 
off to lamps or motors through the building. Fig. 57 shows such a 
center. There may be only one such center in a small installation, 
or in a larger one there may be a main distributing panel and numer¬ 
ous smaller ones at various places in the risk. 

General Rules on Wires. Size. No wire smaller than No. 14 
B. & S. gauge is allowed (except in fixtures and for pendant or flexi¬ 
ble cord) since no smaller size has both the conductivity and also suf¬ 
ficient mechanical strength to stand the strains of installation and use. 

Joints and Splices. 
All joints and splices 
must be made both 
mechanically and elec¬ 
trically secure and then 
he soldered except when 
one of the very few ap¬ 
proved splicing devices 
is used. For general 
wiring, the underwriters 
have never found any 
equivalent for good sold¬ 
ered joints when all the 
possible effects of cor¬ 
rosion, alternate heat¬ 
ing and cooling, vibra¬ 
tion, and mechanical strains are considered. The neatness and 
thoroughness of the soldered joints are two of the best general in¬ 
dications of the excellence of the workmanship on any job. After 
being soldered wire joints must be covered with an insulation equal 
to that at other places on the conductors. This is usually done by 
winding the joints with a good pure rubber tape over which is wound 
a “friction tape” of fabric impregnated with a compound. 

Wires in Walls, Floors, etc. Wires must always be separated 
from walls, floors, timbers, and partitions by non-combustible, non, 
absorptive, insulating tubes such as glass or porcelain and must be 


Fig. 57. Distributing Panel for Lighting Circuit 













UNDERWRITERS’ REQUIREMENTS, 63 

kept free from all contacts with pipes or any conducting material. 
This general rule is established without any reference to the insula¬ 
tion which is on the wires themselves, the idea being that the insula¬ 
tion of the conductors from each other and from other conducting 
materials must be sufficient to furnish the necessary protection in 
case the wire coverings are defective or become injured in any way. 
This principle does not, however, prevent the wires being drawn into 
metal conduits which are specially designed as wire raceways, nor 
can it apply to fixtures in wdnch the wires must be in the metal stems 



Fig. 58 . * Approved Overhead Wiring 


and arms. For such cases special rules are established. Fig. 58 
shows a good example of overhead wiring in which an iron pipe may 
be seen protecting wires up the post, while the wires on ceiling and 
around beams are very well arranged and supported. 

In damp or wet places, the relative arrangement of pipes and 
wires should be such that the wires cannot touch the pipes and so 
that water cannot drop from the pipes on the wires. The subject of 
electrical work in damp places will be considered in another place. 

Carrying Capacity. The Code prescribes the maximum cur¬ 
rent which shall be carried on copper wires of different sizes. 
This table of carrying capacities has been unchanged for many years 

* Courtesy Inspection Department. Associated Factory Mutual Fire. Insurance Companies, Boston, Mass. 






64 


UNDERWRITERS’ REQUIREMENTS 


and was originally based upon an elaborate series of careful experi¬ 
ments. Table III gives the capacity for rubber-covered wires, 
and for all types of wire insulation such as slow-burning and 
weatherproof braids. 

The table for rubber-covered wires is lower than the other be¬ 
cause high temperatures such as might result from a wire carrying 
too much current have a harmful effect on the insulating properties of 
rubber. The table is for indoor work only. It is stated that for any 
given size of wire, a current about three times as great as that given 
in Table III will cause all ordinary insulations to smoke. The table 
does not consider the question of drop as it is called. Thus in Fig. 
59, suppose D is a dynamo supplying current to M, a motor 250 feet 
away from it, and suppose the motor requires 90 amperes at 220 volts. 
From Table III it is seen that No. 2 wire could be used. But 
some power is lost in driving the current through the 500 feet of 
line wire and, if the wire is small, its resistance will be large and so 
more power will be “lost on the line wires.” The part of the 

dvnamo voltage required to 

an AAAPFDF* 

drive the working current over 
the supply wires is called the 
“drop.” Suppose it is pre¬ 
scribed that the drop shall not 
be over 1 per cent of the total 
voltage. One per cent of 220 
volts is 2.2 volts. The current in the line equals the voltage to force 
the current over the line divided by the ohms resistance of the line. 
2.2 


250 FEET 


Fig. 59. Simple Electric Power Circuit 


In this case 90 = 


resistance 


or resistance of the 500 feet of wire 


2.2 

must not be more than — ohms or .024 ohms. From a suitable wire 

table it will be found that a No. 0000 wire will be required. Thus 
the necessity of keeping the loss of power low on the line may neces¬ 
sitate the use of a larger wire than would be needed for safety under 
the underwriters’ rules. 

Constant=Current Systems. The nature of these has already 
been explained on page 13. Such systems are used nowadays almost 
exclusively for street lighting with arc lamps and the voltage runs 







UNDERWRITERS’ REQUIREMENTS 


65 


TABLE III 


Carrying Capacity of Wires* 



Rubber Insulation 

Other Insulations 

i 

B. & S. G. 

Amperes 

Amperes 

Circular Mils 

18 

3 

5 

1,624 

16 

6 

8 

2,583 

14 

12 

16 

4,107 

12 

17 

23 

6,530 

10 

24 

32 

10,380 

8 

33 

46 

16,510 

6 

46 

65 

26,250 

5 

54 

77 

33,100 

4 

65 

92 

41,740 

3 

76 

110 

52,630 

2 

90 

131 

66,370 

1 

107 

156 

83,690 

0 

127 

185 

105,500 

00 

150 

220 

133,100 

000 

177 

262 

167,800 

0000 

210 

312 

21],600 

Circular Mils 




200,000 

200 

300 


300,000 

270 

400 


400,000 

330 

500 


500,000 

390 

590 


600,000 

450 

680 


700,000 

500 

760 


800,000 

550 

840 


900,000 

600 

920 


1 , 000,000 

650 

1,000 


1 , 100,000 

690 

1,080 


1 , 200,000 

730 

1,150 


1 , 300,000 

770 

1,220 


1 , 400,000 

810 

1,290 


1 , 500,000 

850 

1,360 


1 , 600,000 

890 

1,430 


1 , 700,000 

930 

1,490 


1 , 800,000 

970 

1,550 


1 , 900,000 

1,010 

1,610 


2 , 000,000 

J---L 

1,050 

1,670 



*The carrying capacity of Nos. 16 and 18 B. & S. gauge wire is given, but no smaller 
than No. 14 is to be used, except as allowed under rules for fixture wiring. 























66 


UNDERWRITERS’ REQUIREMENTS 


from 2,000 to 3,300 volts. The arc lamps on such circuits are arranged 
so that each line has enough lamps in series to use the available 
dynamo or transformer voltage allowing the necessary margin for 
regulation. 

The high voltages generally employed, call for the very best 
insulation and only rubber-covered wire should be used; all wires in 
buildings must be in plain sight and never encased. The bringing 
of such circuits into buildings is not very general and arc lamps 
designed to’be connected in multiple on ordinary low-voltage circuits 
are much to be preferred. 

There are special rules for series arc-lamp wiring in buildings 
which cover the method of bringing supply wires through the walls, 
provision for a special form of switch at points where the lines enter 
and leave the building, requirement for 1 inch separation between 
wires and the surfaces over which they pass and 8 inches from each 
other and extra protection of all wires by running boards or guard 
strips. The service switch required on constant-current systems 
must be a double-contact switch, that is, it must be so arranged as to 
first place a cross connection or short-circuit on the lines into the 
building and then disconnect these lines from the supply altogether. 
This leaves the circuit unbroken, but cut out of the building. An 
attempt to actually break the circuit would be sure to cause a very 
destructive arc. Series arc lamps must be carefully isolated from all 
inflammable stuff and unless they are of the “enclosed arc” type 
must be provided with screens and nets to prevent the escape of 
sparks from the carbon or melted copper. All connections must be 
made in a most reliable manner and in all series arc work it must be 
remembered that the voltage across any break in the circuit is very 
high and will cause very severe arcing. 

Incandescent lamps are not generally connected to series 
circuits since their use involves an automatic cut-out at each lamp 
which will shunt the current around the lamp in case the lamp be¬ 
comes loose or its filament breaks. These devices are expensive, 
difficult to keep in order and generally undesirable. Formerly, com¬ 
binations of incandescent lamps on series circuits were used, 
consisting of groups of lamps in series or in multiple, but these 
arrangements are not now in use and are forbidden in the rules. It 
is evident that incandescent lamps on series circuits should never be 


UNDERWRITERS' REQUIREMENTS 


67 


allowed on gas fixtures since an arc to the grounded gas pipe would 
be very severe and would be liable to burn through the pipe and 
ignite the escaping gas. 

Constant=Potential Systems. The character of these systems 
has been explained on page 12. Almost all systems for light and 
power in buildings of all sorts are. of this type. The most common 
are 110-volt two-wire direct-current systems; three-wire systems 
with 220 volts between outside wires and 110 volts between the 
neutral and either outer wire; 500- to 600-volt d. c. street and elevated 
systems with “ground return”; 440- to 600-volt a. c. systems for 
motors. In addition to these there are 1,100-, 2,200-, and 3,300-volt 
a. c. power circuits and the so-called a. c. transmission lines at all 
voltages from 1,000 up to 80,000 or 100,000 volts. While occasionally 
a. c. motors are made for direct operation at 1,100 or 2,200 volts, 
in general for voltages above 600 volts, alternating-current trans¬ 
mission lines are employed which are connected to transformers at 
the factories or mills where the power is used and which “step-down” 
the voltage to 440 volts or some other voltage which can conveniently 
be used in the motors and for lighting purposes. Both the primary 
and secondary circuits in this case are constant potential systems. 

In street railway w T ork large machines called rotary converters 
are employed which are driven by the high-voltage alternating 
current from transformers connected to the transmission lines and 
which deliver direct current at about 600 volts to the trolley system. 
Such rotary converters are usually placed in substations so located 
as to conveniently and economically supply the different sections of 
a city. A similar practice is followed in cities where direct current 
is to be furnished for general lighting and power, and the original 
generating station is more or less remote. Such substations come 
under the same rules as generating or dynamo stations. 

All so-called isolated plants, that is, plants in individual factories 
or large buildings, are constant-potential systems also. In the 
underwriters’ rules constant-potential systems are subdivided into 
low-potential systems 10 to 550 volts; high-potential systems 550 to 
3,500 volts; and extra-high-potential systems over 3,500 volts. Of 
these the low-potential systems are of most importance since they 
include the very great majority of equipments for using electricity 
in buildings for light, heat, and power. 


68 


UNDERWRITERS’ REQUIREMENTS 


GENERAL INSTALLATION RULES FOR CONTROLLING AND 
PROTECTING DEVICES 

Switches, fuses, and circuit breakers may all be described as 
arcing devices, that* is, their operation always produces an arc. 
This arc may be small or large but it is impossible to break a circuit¬ 
carrying current without some arc even if it is so small that it is a 
mere spark. The duration and intensity of an arc depends upon the 
strength and voltage of the current, the rapidity with which the gap 
in the circuit is widened, and the design and condition of the arcing 
device, switch or fuse. Dust or inflammable gases may be ignited 
by an arc of sufficient intensity. No arcing device, therefore, should 
be placed near easily ignitible stuff, or exposed to inflammable gases, 
or dust, or flyings of any combustible material. When so exposed, as 
in flour mills, textile mills, etc., all switches and fuses should be en¬ 
closed in dust-tight boxes or cabinets. Open-link fuses are espe¬ 
cially liable to flash violently and throw out molten metal and they 
must, therefore, be given special attention, and should never be 
installed outside of proper cabinets except on switchboards in fire¬ 
proof rooms, such as engine rooms, generating stations, or where 
they will be under constant and expert supervision. Even in ordi¬ 
nary rooms, houses, stores, or factories, where there is no dust or 
combustible flyings in the air, it is much better to have all knife 
switches and all fuses placed in cabinets to prevent accidental short- 
circuits, caused by laying a metal object across the exposed parts. 

Switches immersed in oil are in common use for large currents 
and are quite safe as regards arcing, though the oil involves a certain 
hazard since it is combustible. 

It should be remembered that any switch or circuit breaker 
which is automatic, that is, which is not operated by hand by a 
person at the actual device itself, requires better protection since in 
case of failure the arcing may be severe and no one may be at hand 
to take the needful steps to prevent its starting a fire. Such auto¬ 
matic current-breaking devices including time-switches worked by 
clocks, sign flashers, and the like, should always be enclosed in very 
substantial non-combustible cases or cabinets of ample size and so 
arranged that they are not liable to be left open. 

Switches. The general requirements for service switches have 
already been discussed. While it is true that the service switch 


UNDERWRITERS’ REQUIREMENTS 


69 


and a switch for every motor are the only ones that are absolutely 
required by the rules, still convenience and economy of operation 
naturally call for a number of switches in practically every installa¬ 
tion, and the correct placing of them becomes, therefore, a matter 
of importance. The description of some of the very numerous types 
of switches will be given later in this book, but we consider here the 
general rules for installing all types. 

As a general principle, switches must always be placed in dry, 
accessible places and it is well to group them together so far as pos¬ 
sible for the reason that this will often reduce the amount of wiring 
and also render it easier to use them in case of need. 

Knife Switches. Knife switches consist of copper blades, one 
for each pole, hinged at one end to copper clips or posts and closing 
at the other into other clips. Where such a switch is made to close 




15 



Fig. 60. Single, Double, and Triple Pole Knife Switches 

into clips at only one side of the hinge end it is called a single-throiv 
switch and where the blades can be thrown into clips at either side 
of the hinge it is called a double-throw switch. Single-throw switches 
must always be installed so that gravity will tend to open rather 
than close them since otherwise they might fall and, by only partly 
closing, cause arcs and burning. Double-throw switches may be 
installed so that the throw is either vertical or horizontal as preferred. 
Fig. 60 shows a double-pole single-throw switch and a triple-pole 
double-throw switch correctly placed and a single-pole single-throw 
switch wrongly placed. 

Whenever practicable, knife switches should be so wired that 
the blades will be dead when the switch is open as this leaves less 
exposed live metal and also makes it easier and safer to make any 
repairs or adjustments of the switch blades and hinges. In Fig. 61 
if the supply wires (from the service or dynamo) enter at the top 

















70 


UNDERWRITERS’ REQUIREMENTS 


and the lamps are connected from the bottom of the switch, the 
blades will be dead when the switch is open. If the arrangement is 

reversed, the switch blades will be 
connected to live wires all the 
time, whether the switch is open 
or closed. The illustration also 
shows an excellent type of cast-iron 
cabinet for such a swdtch combina¬ 
tion. 

Surface Snap Switches. These 
are the common porcelain base 
switches, usually round in shape 
and having metal covers with the 
operating handle at the center. 
They are commonly mounted^ on 
side walls and the wires are 
brought into them from the back. 
It is not possible to fasten them very securely to a lath-and-plaster 
wall unless some block is provided for the screws to be driven into. 
For this reason, wherever possible, at all switch or fixture outlets, 
a f-inch block must be fastened between studs or floor timbers flush 
with the back of lathing to hold tubes, and to support switches or 
fixtures. When this cannot be done, wood base blocks, not less 
than j inch in thickness, securely screwed to lathing, must be pro¬ 
vided for switches, and also for fixtures which are not attached to 
gas pipes or conduit. Figs. 62 and 63 show these blocks with the 
wires brought through them and through the lath and plaster in short 
lengths of flexible tubing. The switches can thus be firmly screwed 
to the blocks and the wires connected to them. 


Fig. 62. Fig. 63. 

Proper Arrangement of Wires Passing through Lath-and-Plaster Partition 

If snap switches are used with exposed wiring on cleats, there 
must be a porcelain sub-base under each switch so made that the 
wires will be kept \ inch from the surface wired over. A similar 





Fig. 61. Approved Metal Switch Box 









UNDERWRITERS’ REQUIREMENTS 


71 



sub-base must be used where such a switch is used with wood 
molding, but in this case it may be of hard wooddnstead of porcelain. 
Figs. 64 and 65 show how this is done. 

Flush Switches. These are made to be inserted 
into walls so that only the operating push buttons 
or handle will extend out beyond the surface, and 
are now in very general use. Inasmuch as their 
operating parts are concealed iri the wall they 
should invariably be set into small steel boxes 
through the back of which the wires may enter 
either through lengths of flexible tubing or through 
iron conduit. The same requirement applies to all 
small fittings such as receptacles from which flexi¬ 
ble cords are run to heaters and other portable de¬ 
vices. Fig. 66 is a sketch of such a switch box set 
into a lath-and-plaster wall. The sketch shows the 
box as it would appear from the back of the wall. 

Where it is desired to control the same electric 
lamps from either of two switches at different places, 
what are called three-way switches are installed. 

These are chiefly used in residences, as for the control of hall lights 
from either upstairs or downstairs. Under 
the rules these are classed as single-pole 
switches and are preferably wired so that only 
one main of the circuit is carried to either 
switch. Three-way switches are usually of the 
common round-surface porcelain-base type or 
push-button wall variety. Fig. 67 gives a dia¬ 
gram of the way to connect them. 

Fuses and Circuit Breakers. These may 
be compared with “safety valves” on steam 
boilers, that is to say, they are primarily de¬ 
signed to act in case of an improper condition 
of affairs and prevent by their automatic action 
any serious trouble resulting. Although this 
is the purpose of fuses and overload circuit 
breakers, it is altogether too common for users 
Fig ' 65 ’ Molding 11 W °° d electric current to misuse them, and so de- 


Fig. 64. Exposed 
Wiring on Cleats 


















72 


UNDERWRITERS’ REQUIREMENTS 


stroy the protection intended. It is evident that a fuse which is so 
large that it will not melt until a current passes through it which is 
far too large to be safely carried by the 
wires or other parts of the circuit, is 
worthless. The fusible part of a fuse is 
usually a strip or wire of soft lead or 
zinc of such size that if any considerable 
current over that for which it is designed 
passes through it, it will melt off and so 
open the circuit. Whenever a fuse of the 
proper size for its circuit blows or melts, 
the first thing to be done is to seek out 
the cause, for the operation of the fuse is 
proof that there is, or has been, something 
wrong. Thus if in a house a lighting- 
circuit is properly protected by 6 ampere 
fuses and these fuses blow, one may be 
sure that more than 6 amperes, and, therefore, more than a safe current 
has, for some reason, traversed the wires for a time long enough to melt 
the small fuse strip. Now, unless this excess of current was due to 
some momentary accident, known and recognized as such, the same 
condition that once allowed the unduly large current to flow probably 
still exists, and until this trouble is sought out and remedied, the 
same unsafe condition exists. It is, therefore, very unwise to replace 
the blown fuses with some of larger current-carrying capacity, for 


?W<X> 

SUPPLY 

Fig. 67. Wiring Diagram for Three-Way Switch Circuit 

this is merely reducing the protection without removing the source 
of danger. 

Still worse is it to replace blown fuses by fuses which have been 
filled up with metal, or across, or through which, extra metal strips 
have been fastened in a misguided attempt to keep the fuses from 





Fig. 66. Flush Switch Box in Lath- 
and-Plaster Partition 







































UNDERWRITERS’ REQUIREMENTS 


73 


blowing again. One might as reasonably tie down a steam boiler’s 
safety valve. These principles when thus stated appear very ele¬ 
mentary and self-evident and yet the misuse and abuse of fuses is, 
perhaps, the commonest fault observed in the maintenance of electric 
installations. The only reason that disaster does not always follow 
ignorant or culpable misuse of fuses, is to be found in the fact that 
wires and other parts of the system are installed with a fairly large 
margin of safety. This does not, of course, in any degree justify 
over-fusing circuits or tampering with fuses or other safeguards, 
and insurance inspectors should not tolerate any deviation from 
standard rules for the protection of circuits or fail to demand the 
use of only approved protective devices of proper rating for every 
circuit. Fig. 68 shows some fuses which have been “doctored” in 
ways unfortunately all too common. 

The following excellent statement is taken from the book of 
rules of the Associated Factory Mutual Companies: 



Fig. 68. Typical Examples of “Doctored” Fuses 


Specifications for fuses require that they shall be rated at a certain per 
cent of the maximum current which they will carry indefinitely, as follows: 
link fuses 80 per cent and enclosed fuses 90 per cent. The margin thus pro¬ 
vided between the rating of the fuse and its actual melting point will permit 
the ordinary fluctuations in current without opening the circuit. If fuses 
selected to conform to the above rule are not large enough to carry the load, 
it is evident that the wires also are overloaded, and either the load should be 
diminished or the size of the wire increased. 

Circuit breakers are so sensitive that it is often necessary to set them 
much above the ordinary current to keep them from being constantly opened 
by momentary rises in the current, such as might be caused by starting a motor 
or by a rise in the voltage of the dynamo due to a sudden decrease of load. 






74 


UNDERWRITERS’ REQUIREMENTS 


When this is the case, a fuse may be necessary to protect the wire from a 
steady current above the safe carrying capacity of the wire but below the point 
at which the circuit breaker is set to open. The fuse requires a little time to 
heat, and so does not melt with the momentary rises of current which would 
open the circuit breaker if it were set as low as it would have to be if the fuses 
were not provided. 

It has already been pointed out that “service entrances” must 
be fused, that is, there must be a fuse in each wire where the current 
supply is brought into a building. An exception is made in the case 
of three-wire (not three-phase a. c., however), systems. In these the 
fuse may be omitted in the neutral wire, provided this wire is of 
equal carrying capacity with the outside wires and is reliably grounded, 
since in such a three-wire system the neutral wire cannot under any 
condition carry more current than either of the outside wires; the 
fact that it is grounded adds a certain safety, because it is espe¬ 
cially desirable that the neutral should not be opened unless the 



w GROUND 

Fig. 69. Fuse Arrangement for Three-Wire Circuit 


outside wires are also opened, as might occur if a neutral fuse alone 
blew, or was removed without the others also opening. Fig. 69 
illustrates this point. When the fuse in the neutral at N is in place, 
lamps A can have only 110 volts across them and lamps B the same, 
but if the fuse N is removed, the group A will be in series with the 
group B across the 220 volts of the outside wires, and as there are 
4 lamps at A and only 2 at B, the resistance and volts drop across A 
will be only one-half the resistance and drop across B. Thus A 
will get only one-third of 220 volts or 73.3 volts, and B will get 146.7 
volts. The lamps B will thus burn over bright or even burn out, and 
those in A will be dim. Of course, if there were the same number of 
lamps at A as at B, it would make no difference whether the neutral 
wire were open or not, but when the system is unbalanced as in Fig. 
69 it is better to avoid having the neutral opened and, therefore, 












UNDERWRITERS’ REQUIREMENTS 


75 


the fuse in it is often omitted, and the wire carried through unbroken. 

Beyond the service entrance fuses and switch, the circuits are 
usually divided; thus in a house several circuits of smaller wire will 
lead to the lights on the different floors, or in a larger building or a 
factory, sub-mains will be carried up to the distributing centers 
through the building from which in turn smaller wires will branch 
out. The proper proportioning of these cables and wires for the 
economical distribution of current is a problem for the electrical 
engineer. The point to be noted here is that fuses or breakers must 
be placed at every point where a change is made in the size of wire 
unless the fuse next back will also protect the smaller wire. The 
rated capacity of fuses must not exceed the allowable carrying capacity 



of the wire as given on page 65, and circuit breakers must not be set 
more than 30 per cent above the capacity of the wire unless a fuse 
is also used when it may be set 100 per cent higher. Fig. 70 shows 
the sizes of wire and the fuse arrangement for a typical case. The 
arc lamps require 30 amperes, the 20 incandescent lamps 10 amperes, 
and the motor has a full load current of 20 amperes. The motor, 
however, must have leads for 25 per cent above full load current or 
25 amperes or No. 10 wire. Allowing 60 amperes for the total normal 
current the service fuses at A might be 60 amperes and the wires 
next beyond them No. 4. The other wire sizes are shown and the 
fuses would have to be placed as shown and have the following 

























76 


UNDERWRITERS’ REQUIREMENTS 


ratings: B, 30 amperes ;C, 6 amperes; D, 30 amperes; E, 20 amperes; 
and F, 6 amperes. The 6-ampere fuses are required at C and F, 
because by a special rule mentioned below this is the limit for fuses 
for branch circuits at 110 volts supplying incandescent lamps. A 
pair of fuses is also required for each arc lamp and these would in the 
case shown usually be of a 10-ampere rating. If circuit breakers 
were substituted for the motor fuses at E they should be set to open 
at not over 30 per cent above the capacity of No. 10 wire or about 31 
amperes. 

It will frequently be found necessary to provide cut-outs where 
taps are taken from large mains. In such cases, if the clamps on the 
cut-outs are not sufficiently large and strong to give a firm and secure 
connection, a short length of smaller wire may be soldered to the 
main wire and then carried direct to the cut-out, which should be 
located as near as possible to the point of connection with the mains. 
Special care should be taken to guard these leads from accident as 
they may not be properly protected by the fuses in the main circuit. 

Fuses are always installed in pairs (or sets of 3 on three-wire 
circuits), so that each side of the circuit is fused for the reason that a 
“ground” or “cross” might occur so as to cause a large current to flow 
over a path not including any fuse. Furthermore, fusing both sides 
gives a much greater factor of safety and insures protection under 
all conditions. 

In installing incandescent lamps, fuses must be so placed that 
no set of lamps requiring more than 660-watts power will be depend¬ 
ent on a single pair of fuses. Some city rules state this in terms regu¬ 
lating lamp sockets or receptacles, and limit the number of sockets 
to 10 or 12 since this amounts to about the same thing. The purpose 
of this rule is to secure such a subdivision of the fuses that no very 
large currents can flow for any long time over any part of the small 
wiring without opening a fuse and thus the effects of a short-circuit 
or other accident will be very much minimized. Suppose we had a 
long line of No. 10 wire protected by 25 ampere fuses on a 110-volt 
circuit, and 50 incandescent lamps in multiple on this line with no 
other fuses. Suppose also that only one of the lamps were burning. 
Now if this lamp were hung on a flexible cord and a “short-circuit” 
occurred on the cord or in the lamp socket, we should have to wait 
until the short-circuit became severe enough to allow about 30 


UNDERWRITERS’ REQUIREMENTS 


77 


amperes of current to flow before the 25-ampere fuses would open. 
(All fuses will carry, for a short time, more than their rated currents.) 
But 30 amperes at 110 volts may cause an arc quite intense enough 
to set fire to the cord or cause molten metal to drop from the over¬ 
heated socket. If the lamps were on a branch circuit of No. 14 wire 
protected by 6-ampere fuses, these fuses would probably melt before 
serious harm were done. Thus it may be seen that it is wise to sub¬ 
divide the lamp circuits and protect each circuit with small fuses. 
In exposed wiring in large mills an exception is made permitting 
incandescent-lamp circuits with 25-ampere fuses to be used provided 
each lamp is protected by a very small fuse placed in a ceiling rosette. 
This is allowed to prevent running an excessive amount of wiring 
through large rooms where the crowded wires might be themselves 
a source of danger. It was formerly the custom to place small fuses 
in the canopies of fixtures or ceilings and side walls but they were 
always troublesome and dangerous to an extent that did not offset 
the slight extra protection they afforded and all such “bug” fuses 
have long been forbidden. 

Enclosed fuses, plug and cartridge, are allowed by the Code rules 
to be installed without cabinets except in dusty or linty places but 
some municipal ordinances require all fuses to be in cabinets. While 
this is safer, there would be a tendency to use the open-link lead fuse 
everywhere if cabinets were universally required since they are much 
cheaper than enclosed fuses, and this is a tendency not to be en¬ 
couraged. If proper locations are chosen for enclosed fuses and if 
only approved fuses are used, there is little hazard under ordinary 
conditions. Enclosed fuses either of the cartridge or the plug type 
and of makes having the underwriters’ approval will open the circuits 
for which they are rated with practically no explosive action and 
without emitting flame or molten metal even on heavy short-circuits. 
Even so, however, they should never be placed near combustible 
material. 

Electric Heaters. Under this heading are included all devices 
in which use is made of the heat developed by the current (usually 
by causing it to pass through coils of wire). These are electric 
pressing irons, air and water heaters, toilet articles, such as curling 
irons, cooking devices of all sorts, and a vast and constantly increas¬ 
ing variety of domestic and industrial appliances. All these present 


78 


UNDERWRITERS’ REQUIREMENTS 


the same hazards as other heaters of equal capacity, except that 
the dangers of open gas flames and of matches are eliminated, and 
they all require the same precautions in use and in their installation. 
Each should be protected by fuses either on the device or preferably 
in the branch circuits supplying them and must be controlled by 
separate switches so made as to indicate whether the current is “on” 
or “off.” These must be double-pole switches if more than 660 w 7 atts 
of energy is required. In general, such heaters should never be “built 
in” but should be in plain sight. However, for many industrial pur¬ 
poses electric heaters are constructed as parts of tools or machines 
and when well made and used with due care are not more hazardous 
than other methods of heating. 

Portable heaters are in general more dangerous than stationary 
ones since the latter may be safeguarded by suitable heat-resisting 
material placed between the device and its surroundings, such as 



Fig. 71. Effects of Careless Use of the Electric Iron 


sheets of tin or steel with an air space between them or by alternate 
layers of sheet steel and asbestos with a similar air space. 

The electric flatiron is perhaps the cause of more trouble and 
danger from fire than any other form of heater. The temperature 
of the iron required for ironing damp fabrics is necessarily high (at 
least 500° F. is common) and if the iron is left with current on and 
is not in use it will become red-hot in from ten to twenty minutes. 
If it has been left on a table or on clothing a fire is almost inevitable. 
Fig. 71 shows a cloth-covered board burned by electric irons. Few 
irons have any automatic cut-off to guard against such an accident 
and the fact that many such irons are used by persons not familiar 
with the possible danger, makes these devices rather hazardous. 

It is often desirable to connect in multiple with the heaters and 






UNDERWRITERS' REQUIREMENTS 


79 


between the heater and the switch controlling same, an incandescent 
lamp of low candle power, as it shows at a glance whether or not the 
switch is open, and tends to prevent it being left closed through 
oversight. An approved stand, of a pattern such that the iron may 
be safely left on it even with the current on, should be used with every 
electric pressing-iron. The ordinary plain iron stand for cast flatirons 
is not adequate as it will become hot enough to set fire to a table. 
It should be remembered that stove-heated irons get cooler when 
taken from the stove, while an electric iron will get hotter and hotter 
if left connected to the circuit and 
not used. Fig. 72 shows an iron 
properly installed with indicating 
switch, pilot lamp, and stand. 

Portable heaters, if they re¬ 
quire over 250 watts, should be 
furnished with approved “heater 
cord” which consists of stranded 
copper conductors with a thin 
rubber and a thick asbestos yarn 
covering over each with a good 
braid over all. In factories and 
shops where a large number of 
flatirons or rather portable electric 
heaters are used, the circuits lead¬ 
ing to them should be so arranged 
and provided with switches that 
any department or tier of benches Fig. 72 . Properly installed Electric 

1 , rv 1 , • • l Iron Circuit 

can be cut oft when not in use and . 

pilot lamps in conspicuous places should be provided to call attention 
to the fact that the circuit is closed to the heaters. Domestic cooking 
devices electrically heated, are, in general, fairly safe as made by well- 
known manufacturers, and are preferable from the viewpoint of fire 
hazard to similar appliances heated by gas or by alcohol lamps. 

The use of flatirons or other heaters on ordinary lighting circuits 
is undesirable since they often require more current than can be taken 
through the proper fuses for such lamp circuits. It is very desirable 
that special circuits be run of large wire properly fused and with large 
capacity fittings and that all heating and small power devices such 

















80 


UNDERWRITERS' REQUIREMENTS 


as washing and vacuum cleaning machines be supplied from such 
special circuits. The ordinary wall or ceiling fixtures and the lamp 

sockets attached are very ill suited for 
connecting portable heaters because of 
their lack of current-carrying capacity; 
and, too, because they are not me¬ 
chanically strong enough to with¬ 
stand the comparatively rough usage 
to which they are inevitably subjected. 



f fijf| 


ifV ;i 




Fig. 73. Punctures in Gas Pipes 
from Short-Circuits 


sible may be maintained. 


FIXTURES AND FIXTURE WIRING 

Fixture Details. Electric fixtures 
are of two types known as straight 
electric and combination, the former 
carrying only electric lamps and the 
other having both electric and gas 
lights. The chief parts of a fixture are 
the canopy at ceiling or wall, the stem, 
the body, the arms, and the sockets. 
Most fixtures consist of plain iron pipe, 
often of small size, threaded into 
special castings in the body or central 
ball with arms branching from the 
body. Over this pipe body is a casing 
of brass pipe in pressed or spun forms. 
In straight electric fixtures the wires 
are usually drawn through the iron 
pipes, but in combination fixtures the 
gas runs in the pipes and the wires lie 
along the outside of the pipes between 
them and the casing or are drawn 
through cored holes in cast-brass arms. 
The necessarily small spaces and chan¬ 
nels for wires in fixtures and the slight 
amount of insulation which the wires 
can carry, require very careful work 
in order that the best conditions pos- 
The pipes in which wires are drawn should 



UNDERWRITERS’ REQUIREMENTS 


81 


be carefully reamed at ends and all sharp angles, burrs, and corners 
on which wires may be injured should be carefully rounded off. These 
details should be attended to by the fixture maker but are often neg¬ 
lected, resulting in such accidents as are shown in Fig. 73, where the 
current has burned holes through the gas pipe and ignited the gas. 
Nothing but rubber-covered wire having a rubber wall, and no wire 
smaller than No. 18 B. & S. gauge, should be used. Stranded wire 
is preferable to solid wire. A rubber insulation Fi-inch thick is 
permitted on No. 18 wire for fixtures, but all No. 16 wire and all 
flexible cord if used for fixture work should have at least 32 -inch 
wall. Fixtures are not allowed on circuits of over 300 volts. 

Insulating Joint. On lath-and-plaster ceilings and walls where 
steel outlet boxes are not used, fixtures are usually fastened to some 
form of “crowfoot,” a small tripod casting into which the stem of 
the fixture is screwed. Combination fixtures are screwed on the pro¬ 
jecting nipple of the gas pipe. When fixtures are thus supported 
on gas pipes or when they are attached to any grounded metal work 
of a building or are on walls or ceilings of plaster on metal lathing, 
an approved “insulating joint” must be inserted between the fixture 
and its support. An insulating joint is a coupling, the two ends of 
which are reliably insulated from each other by some substance, 
usually mica. For combination fixtures there is a hole for the gas 
through the center of the joint. Such joints are required to be made 
of materials which will not be affected by the gas; no soft rubber is 
allowable and they must be capable of withstanding a voltage test 
of 4,000 volts a. c. between the two ends. Such insulation is required 
because the fixture wire is necessarily poorly insulated and liable 
to permit the conductors to become “grounded” on the fixture at one 
or more points. The insulating joint prevents such a failure in the 
fixture from causing current to flow to the earth in case the circuit is 
either purposely or accidentally grounded at other places. It also 
prevents voltage sufficient to puncture the fixture wiring from arcing 
across broken insulation to the fixture and so to the ground. 

Canopy Insulators. Since the canopy or bell covering the base 
of the fixture at the ceiling or wall is in electrical connection with 
the fixture stem, it also must be insulated at its upper edge from the 
wall wherever an insulating joint is required. Canopy insulators 
take the form of molded rings of composition, fiber rings riveted to 


82 


UNDERWRITERS’ REQUIREMENTS 


the canopy edge or insulating linings and flanges of micanite or other 
suitable material. Fig. 74 shows a correctly mounted combination 
fixture with insulating joint and canopy insulator. The cut also 
shows the pieces of flexible tubing in which the wires are separately 
encased from the last porcelain support, through the ceiling and to 
a point below the insulating joint. B in the figure is a piece of 
insulating tubing which should be placed about the gas pipe above 
the insulating joint. In case the wires were in steel conduit a steel 
outlet box would be placed in the ceiling, the gas pipe would enter 
through a central hole in the back and the conduit through another 
hole at one side. In such cases the flexible tubes would not be used 

but the canopy insulator and 
the insulating joint would still 
be required. Most trouble from 
fixtures occurs in the canopy 
from poorly made wire joints and 
crowded or jammed wires which 
gradually give way until finally 
an arc is formed and the wire 
coverings are ignited, Fig. 75. 
Canopies are often made too 
shallow and too small and work¬ 
men are often careless in connect¬ 
ing fixtures. 

Sockets and Receptacles. 

These are the devices into which incandescent lamps are put and 
are today made in a great variety of forms. The most common is 
the familiar brass shell socket either with a key switch or keyless, 
screwed on the ends of fixture arms or hung on flexible cords. Where 
a lamp holder is desired to be fastened rigidly to walls or ceilings, re¬ 
ceptacles are used and this in general constitutes the distinction 
between receptacles and sockets. For outdoor use or in damp 
. places weatherproof sockets or receptacles should be used. These 
usually have porcelain or composition outer shells, with connecting 
wires sealed in or with some form of encased terminal so placed 
and covered as to keep moisture from exterior and interior. 

It is good practice to use only porcelain sockets in locations 
where a person might touch them while at the same time in contact 



Fig. 74. Correctly Mounted Combination 
Fixture 















UNDERWRITERS’ REQUIREMENTS 


- 83 


in any way with any grounded metal work. Thus in bath rooms if a 
person attempted to turn on a lamp by means of the key of a brass 
covered socket while at the same time he was touching a water pipe 
or faucet, he might receive a painful or dangerous shock if any portion 
of the electric circuit were grounded. While such a shock would 
not be given, if everything were in proper condition, still there are 



Fig. 75. Result of Defective Fixture Insulating Joint 


numerous instances where persons have been killed and it is cer¬ 
tainly wise to prevent even the chance of such an accident by either 
using all porcelain sockets or by putting the sockets out of reach, 
and controlling the lamps by means of a switch on a side wall. This 
practice is especially to be commended where the lamps are supplied 
with a, c. transformers the primaries of which are supplied by circuits 









84 • 


UNDERWRITERS’ REQUIREMENTS 





Fig. 76. Enclosed Incandescent Lamp a 
Precaution Against Explosive Vapor 


of voltages of 1,100, 2,200, or higher values. The voltages of 110 or 
even 220 either d. c. or a. c., which are almost universally used for 
incandescent lighting indoors, are not of themselves at all liable to 

injure persons, but if there is a 
fault in the transformer the high 
primary voltage may get into the 
house over the secondary lines 
and such a circuit will then be 
distinctly dangerous to life. 

In rooms where inflammable 
gases may be present, as the re¬ 
sult of some manufacturing proc¬ 
ess or otherwise, the incandes¬ 
cent lamp and its socket must 
be enclosed in a vapor-tight globe 
and supported on a pipe hanger, 
wired with approved rubber- 
covered No. 14 wire soldered directly to the circuit wires. Even the 
minute spark caused by breaking a 16-candle-power lamp has been 
known to set fire to vapors such as gasoline and air in the proper 
mixture and the reason for taking every precaution against sparks 

where such vapors exist, becomes 
very apparent. Eig. 76 shows 
a lamp so enclosed and sup¬ 
ported. 

In damp or wet places weath¬ 
erproof sockets are required 
and these must either be made 
upon fixtures or hung as shown 
in Fig. 77. Here stranded No. 
14 rubber-covered wires are 
shown passing from the socket to 
the circuit wires to which they 
are soldered. The socket and 
lamp must not hang direct on the soldered joints but some other 
support must be supplied as by holding the socket wires under one 
of the porcelain cleats. The ordinary brass shell socket is a fairly 
standard device as now furnished by the best makers, but it is of 













UNDERWRITERS’ REQUIREMENTS 


85 


necessity small, and not very strong. It is decidedly better 
never to use such a socket except for an incandescent lamp and 
where an outlet is desired from which to take current for portable 
heaters, fans, and the like, to provide special wall or floor receptacles. 
This insures adequate current-carrying capacity and avoids mechan¬ 
ical injury to the comparatively frail sockets. 

Flexible Cords. There are two chief classes of flexible cords, 
the plain twisted pairs and the various types of reinforced cord for 
portable use and where extra protection and strength is needed. It 
will be observed that the use of any type of flexible cord constitutes 
an exception to the general rule that conductors must be well sepa¬ 
rated. From one point of view it seems inconsistent to require wires 
to be well separated in walls, floors, and partitions and then permit 
the two conductors of a cord to 
be twisted closely together, with 
only a thin rubber insulation and 
a cotton braid on each. It is, in 
fact, a concession made to the 
necessities of the case and it 
cannot be denied that flexible 
cords may be and often are the 
weakest part of an ordinary wir¬ 
ing installation. It thus be¬ 
comes at once evident why very 
definite limitations must be made 
in the use of cords. Fig. 78 shows 
types of flexible cords. 

The common “twisted pair” cord consists of stranded copper 
conductors having a total carrying capacity equal to that of No. 18, 
16 or 14 B. & S. gauge solid wire. No conductors smaller than No. 18 
are allowed even for very small currents in order that the mechanical 
strength may not be too little. (It should be noticed also that No. 
18 and No. 16 wires are permitted only for flexible cord and for fixture 
wiring, No. 14 or larger being required everywhere else.) 

The copper strands are twisted or cabled together and wound 
with a wrapping of cotton thread both to keep the rubber from direct 
contact with the copper which it tends to corrode and to prevent a 
broken strand of wire puncturing the insulation. 



a b c 


Fig. 78. Types of Flexible Cords 




86 


UNDERWRITERS’ REQUIREMENTS 


The rubber insulation should be 32 -inch thick on No. 18 and 
No. 16, and A-inch on No. 14 cords and over each should be a fairly 
close braid of cotton thread. It is apparent that the finished cord so 
made with its two conductors twisted closely together does not afford 
any very great protection against a short-circuit resulting from broken 
or worn insulations and braid, and that the rubber and cotton braids 
supply a very fairly good fuel for any flame which is started. 

The foregoing is not intended to lead to the conclusion that 
flexible cords should not be used, but rather to point to the reason why 
their use should be restricted and why they are open to objections not 
applying to ordinary separate and fixed wires. 

Flexible cord should never be used as a substitute for regular 
wiring provided does not give outlets at the 
proper places it should be changed in a proper 
and reliable manner. The common practice of 
festooning flexible cord along walls and ceilings 
and even through doorways and walls is to 
be strongly condemned as an unsafe and wholly 
inexcusable misuse of material. The use of 
flexible cord is limited to 300 volts. It should 
not be used to support lamp clusters as they 
are not capable of being secured well enough 
under binding screws to hold any considerable 
weight. The ordinary cord should be used only 
to hold lamps which under all usual conditions 
hang freely in air and which are not likely to 
be moved sufficiently to come into contact with 
surrounding objects. In brief, this cord is for 
“pendant” use only as its common name implies. The use of pen¬ 
dant cord for reinforced cord is a very common fault. Ragged half- 
broken lamp cord lying about on floors and looped over all kinds of 
supports constitutes a very common defect in installations, otherwise 
perhaps in fair condition. “Flexible cord fires” are as common as 
might be expected from the frequent misuse of this form of wire. 

For all portable work including those pendants which are liable 
to be moved about, some form of reinforced cord should always be 
employed. In these the ordinary pendant cord is covered either 
with an outer rubber jacket and a stout “over-all” braid or where 


fixed wiring. If the 



Fig. 79. Wire Guard for 
Portable Lamp 



UNDERWRITERS’ REQUIREMENTS 


87 


the exposure to injury is less (as in offices and residences) with an 
outer woven braid only. These reinforced cords are far from in¬ 
destructible and should be frequently inspected and renewed before 
an accident makes it absolutely necessary. The use of all kinds of 
flexible cord in show windows is expressly forbidden by the Code, 
because it has been found that it is subject to exceptionally hard 



Fig. 80. Results of Careless Handling of Portable Lamps 

usage there, and more especially because of the practical certainty 
that it will be used as a support for window decorations or goods of 
inflammable nature which have often been found 'pinned to it or 
supported by wires strung across it. A special kind of cord having 
a metal armor is, however, allowed in show windows. 




88 


UNDERWRITERS’ REQUIREMENTS 


In connecting flexible cords to sockets, rosettes or other devices, 
special attention should be paid to seeing that all the small copper 
strands are well tucked in under the heads of binding screws and 
that stray ends are not left sticking out to cause short-circuits. At 
all places where cords pass out of sockets or other fittings, smooth, 
well-rounded insulating bushings should be provided and a knot 
should be ‘tied in the cord inside the socket cap or rosette, so that 
this knot will take the strain from 'the binding screws. Several 
special little fittings for this purpose are also on the market as sub¬ 
stitutes for the knot. Pendant lamps wherever exposed to injury 
or at all liable to be brought into contact with inflammable material, 
and all portable lamps should be provided with substantial wire 



Fig. 81. Hole Burned by Incandescent Lamp Bulb 

guards such as are shown in Fig. 79. Figs. 80 and 81 show the results 
of carelessness with hanging or portable incandescent lamps. 

Arc Lamps on Constant=Potential Circuits. Each lamp, or each 
series of lamps, should be separately fused and the branch conductors 
should have a carrying capacity about 50 per cent in excess of the 
current required to provide for the heavy current required by the 
lamp in starting or when the lamp carbons accidentally become stuck. 
If this were not done it would be necessary, generally, to over-fuse 
the wires, which is objectionable. 

Arc lamps are of many patterns but all of them contain resist- 













UNDERWRITERS’ REQUIREMENTS 


89 



ances or regulators which become very hot and, therefore, all the parts 
of the lamp and its case must be of non-combustible material and 
must be treated as sources of heat, that is, they must be installed 

well away from all inflam¬ 
mable stuff. The globes and 
netting about the carbons 
must be used in all cases. In 
general the resistances and all 



Fig. 82. Exterior and Interior View of Enclosed Arc Lamp 


other accessories of the lamp except the controlling switch should 
be contained in the lamp case itself. Fig. 82 shows a modern type 
of enclosed arc lamp and its mechanism. 

The “flaming arc” lamps now in common use call for the same 
precautions as the older patterns. In dusty or linty places special 



90 


UNDERWRITERS’ REQUIREMENTS 


precautions must be taken to prevent the accumulation of lint, etc., 
either on hot resistances, inside the lamp or on the switch usually 
furnished on the lamp. In general the same rules apply to mercury 
arc lamps as to the carbon arc lamps, except that the former do not 
present the hazards due to the hot carbon points. 

TRANSFORMERS IN BUILDINGS 



Except in central stations and substations, an outside location 
for transformers is always preferable and the underwriters do not 

allow any oil-cooled trans¬ 
formers in buildings except 
by special permission. This 
is because of the danger 
from the oil which may be 
boiled over or set on fire in 
case the transformer becomes 
overheated. 

Air-cooled transformers 
having the highest voltage of 
both primary and secondary 
under 550 volts may be in¬ 
stalled inside buildings if the 
case is kept at least one foot 
from combustible material or 
separated from such material 
by being mounted on a suit¬ 
able slab of slate or marble. 
Transformers sometimes be¬ 
come very hot in case of a partial or complete burn-out of the coils 
or from overloading and, therefore, should never be mounted di¬ 
rectly against wood posts, beams, or walls. Fig. 83 shows a com¬ 
mon type of oil-cooled transformer. 


Fig. 83. Common Type of Oil-Cooled Transformer 




















. 

. 

























TESTING LABORATORY SHOWING ELECTRICALLY DRIVEN CENTRIFUGAL FIRE PUMP, UNDERWRITERS’ LABORA¬ 
TORIES, CHICAGO 




































UNDERWRITERS' REQUIRE¬ 
MENTS 

PART II 


INSTALLATION OF WIRES IN BUILDINGS 

The proper choice of wires and their safe installation con- 
stitutes the most important part of all electric equipment from the 
viewpoint of the fire hazard. It has already been stated that in all 
electric work, conductors, however well insulated, should always be 
treated as bare, and from one point of view, it may be said that the 
value of conductors as regards safety lies in their insulation rather 
than in the copper, for if we assume that a wire of adequate carrying 
capacity is chosen for a given purpose, there remains only the choice 
of a suitable covering or insulation on the wire and a reliable and work¬ 
manlike method of placing it. 

No one material has yet been produced which has every desir¬ 
able property as a covering and insulation of electric wires and 
cables. Among the desirable properties of a wire covering are 
elasticity, flexibility, waterproof ness, good insulating quality and 
resistance to voltage strains, resistance to effects of changing tem¬ 
perature, acids, vapors, etc., and permanence. All of these prop¬ 
erties are possessed by rubber in greater or less degree and, all told, 
to a greater degree than any other material. The properties of 
rubber-covered wires will be treated at greater length in another 
place, the foregoing statement being made here to emphasize the 
reason why rubber-covered wires are used almost exclusively in all 
inside wiring. 

“Slow burning” wire is a copper conductor covered with three 
closely woven cotton braids saturated with a fire-resisting compound. 
Its use is limited to places where rubber is liable to be rapidly injured 
by high temperatures. Its insulating value is slight and it is not 
capable of resisting moisture. 



92 


UNDERWRITERS’ REQUIREMENTS 


“Weatherproof” wire consists of a copper conductor covered 
with three braids saturated with a moisture-proof compound. Its 
insulating value when new is low (much less than rubber) and it is 
very inflammable. Its use is practically confined to outdoors. 

In the following sections it may be assumed that all references 
to wire mean “rubber-covered” wire. 

Classification and General Principles. There are two classes of 
wiring which may be named for convenience: enclosed wiring and 
non-enclosed wiring. 

Wires run on insulators such as cleats and knobs exposed on 
walls and ceilings or on knobs and through tubes concealed in floors 
and walls are the chief types of non-enclosed wiring. It will be 
observed that the distinction consists in the presence or absence of 
special wire-ways or channels for the wires. Wires must not be laid 
in plaster, cement, or similar finish, because such materials may 
contain either alkalies or acids which will injure the insulation and 
corrode the copper. Wires must never under any circumstances be 
fastened with staples because of the probable injury to the wire 
coverings, the insecure fastening obtained, and the possibility of 
such staples affording a path between wires in case twin conductors 
are used. 

Twin wires must never be used except in conduit or where flexible 
conductors are necessary. The nearness of the two wires, on the 
opposite sides of the circuit, renders twin wire of any description 
somewhat more liable to failure and an injury to one wire generally 
involves an injury to both with resultant certainty of a short-circuit. 
The added safety of keeping the wires of a circuit separated is lost 
in twin wires. 

In any scheme of wiring it is essential that all electric wires be 
installed so that they cannot come into contact at any point with 
any materials other than those expressly intended for their enclosure 
or support. This means that they should be kept absolutely free 
from contact with gas, water, or other piping, and from all metal 
work of any description unless it be that of piping, boxing, or molding 
provided as wire enclosures. Wires should also be installed so as 
not to touch woodwork or other combustible material, even if such 
material is not a good conductor. This general principle finds an 
exception in the case of wood molding. 



UNDERWRITERS’ REQUIREMENTS 93 

Contact of wires with metallic substances may, in case of a failure 
of the insulation, permit dangerous arcs, short-circuits, or grounds, 
while contact with wood or combustible material is objectionable on 
account of setting fire to it in case of overheated wires or from leakage 
due to the presence of moisture on materials which when dry would 
be good insulators. These are, therefore, the general principles of 
wiring under the established rules and will be illustrated in the dis¬ 
cussion of the various classes of wiring which follow. 

Open Work in Dry Places. Wires in open work may be either 
rubber-covered, slow-burning, or—special and now little used wire 
having a weatherproof braid covered by a slow-burning braid—but 
as a matter of fact only rubber-covered wire is used to any extent 


Fig. 84. Large Feeder Wires Exposed on Insulators 

and it is much to be preferred except in exceptionally hot places as 
over steam boilers, where rubber insulation will deteriorate very 
rapidly. The rubber-covered wire used for open work has a single 
braid over the rubber. The chief advantages of open work are its 
cheapness and its accessibility. The latter may be of great advantage 
in cases where frequent changes and additions are likely to be re¬ 
quired or where renewals of wire are frequent because of- peculiarly 
unfavorable conditions such as exist in packing houses. 







94 


UNDERWRITERS’ REQUIREMENTS 



--- mil, nutr 

Porcelain Two-Wire Cleat 



Open work finds its chief use in mills and factories and for large 
conductors which it is especially difficult and expensive to enclose in 

conduit. Fig. 84 shows 
an example of a set of 
large feeders run exposed 
on insulators. It should 
be noted that such a 
large group of heavy 
cables covered with the 
inflammable braid and 
rubber insulation fur¬ 
nishes a very considerable amount of fuel for fire and the necessity 
for excellent spacing and reliable fastening is obvious. The heavy 
porcelain blocks carried in metal frames as shown in the illustra¬ 
tion are of an approved type. In all open w^ork, wires or cables 
must be rigidly supported on non-combustible, non-absorptive 
insulators. Formerly wood cleats were used but these are now 
obsolete and have been replaced by porcelain. Where the voltage 
is less than 300 volts, wires must be separated from each other at 
least 2\ inches and from the surface wired over at least \ inch in dry 


places—in damp places at least 1 inch. For voltages from 301 to 
550 volts, the limit for “low-potential systems,” the wires must be 
kept 4 inches apart and 1 inch from the surface wired over. 

The neutral wire of an Edison direct-current three-wire system 
(110-220 volts) may be placed in the center of a three-wire cleat 




which will keep the two outside wires 2\ inches apart. Fig. 85 shows 
the form of a porcelain two-wire cleat and Figs. 86 and 87 show good 










































UNDERWRITERS’ REQUIREMENTS 


95 


forms of one-wire cleats for heavier conductors. No. 6 wire is about 
the largest which should be installed in cleats like those in Fig. 85, 
and for cables larger than No. 0000 B. & S. gauge, some form of iron 
rack for the insulators is desirable in order to secure the needed 
mechanical strength and rigidity. 

The rigid supporting necessary for open wiring requires under 
ordinary conditions along flat surfaces, supports at least every \\ 
feet. This distance should be decreased wherever wires are liable 
to be disturbed especially if the wires are small. The following 
comment is from the Rules of the Associated Factory Mutual Fire 
Insurance Companies: 

The proper distance between insulators depends largely on the sur¬ 
roundings. In places where ceilings are low, or where belts, shafting, or other 



Fig. 88. Wiring on Ceiling Showing Use of Strain Insulators 

machinery may require frequent attention, insulators should be placed every 
few feet, in order to prevent the wires from being displaced by careless or 
unavoidable blows from workmen. On the other hand, with a high ceiling 
and no chance of derangement, a greater distance would be allowable. 

The whole idea is to so rigidly secure the wires that they cannot come 
in contact with each other or any other conductors, if loosened by shrink¬ 
age of timbers and floors or by careless knocking. 

Special methods must be followed at corners and in wiring over 
broken surfaces as on ceilings of mill-constructed buildings, big. 
88 show r s a use of “strain insulators” in making a turn on a ceiling. 
These are insulating balls having rings set in each side, the wire 
being looped through one ring and the hook on the beam through 
the other. Turnbuckles may be used to keep the wires taut. Ordi- 





















































































































































96 


UNDERWRITERS’ REQUIREMENTS 



Fig. 89. Approved Method of Carrying Wires 
Around Beams 


nary cleats will not hold heavy wires at corners. With conductors 
of No. 8 B. & S. gauge or over, it is not necessary to “break around” 

„ beams but smaller wires should 
be carried around the beams 
as shown in Fig. 89. The 
cleats on the ceiling should be 
set off from the timbers 3 or 4 
inches. If they are closer, the 
shrinkage of the timber or 
rough usage is liable to bring 
the wires into contact with the timber. On the other hand if the 
distance is greater, the wires are too much exposed to injury from 
brooms, ladders, and the like. With this arrangement any slack 
wire can be taken up by 
moving the cleats a lit¬ 
tle nearer the corner. 

Where beams are widely 
spaced some such method 
of support as that shown 
in Fig. 90 should be fol¬ 
lowed. Fig. 91 shows a 
less desirable method of 
support. In low ceiling rooms where wires are exposed to mechani¬ 
cal injury, wood guard strips (see Fig. 92) may be used to ad¬ 
vantage. Where wires pass through partitions or walls they must 
be protected by tubes of porcelain or iron pipes lined with flexible 
tubing. 

Wires on side walls must be protected from injury to a height of 
at least 5 feet from the floor either by wood boxing or by iron pipe as 

£Z 



Fig. 90. Method of Running Wires between Widely- 
Spaced Girders 


I 



m 

1 



if 


-do- 


e 


Fig. 91. Method of Avoiding Sag between Widely-Spaced Girders 

shown in Figs. 93 and 94. When iron pipe or conduit is used the 
insulation of each wire must be reinforced by flexible tubing extend- 












































UNDERWRITERS’ REQUIREMENTS 


97 



mg from the insulator next below the pipe to the one next above it 
(see Fig. 93). For alternating current both wires of the circuit must 
be in the same pipe. 

OpenWiring in Damp Places. 

The installation of electric wires 
in places exposed to dampness 
presents some peculiar difficul¬ 
ties requiring special methods. 

A film of water such as may be 
formed by steam or otherwise 
by condensation is a very fair 
conductor of electricity, and may reduce or even entirely destroy 
insulation which would be quite adequate in dry places. Acid or 
alkaline fumes or vapors are also good 
conductors in some cases and in addition 
they are liable to injure both the insula¬ 
tors and the copper of electric wires. 


Fig. 92. Diagram Showing Use of 
Guard Strips 



Fig. 93.* Wood Boxing as Protection 
for Wires on Side Walls 


Fig. 94 * Protecting 
Wires on Walls by 
Iron Piping 


In paper mills, breweries, soap factories, packing houses, dye 
works, and cold storage rooms special pains must be taken to insure 

* Courtesy Inspection Department, Associated Factory Mutual Fire. Insurance Companies, Boston, Mass. 

























































98 


UNDERWRITERS’ REQUIREMENTS 


permanence and reliability of all electric fittings and appliances. 

The Code does not prescribe at length precautions to be taken 
in damp or otherwise exceptionally troublesome places, but merely 
specifies for open wiring that only rubber-covered wire be used and 
that the separation between wires shall be at least 2\ inches for 
voltages up to 300 volts (4 inches for higher voltages) and that all 
wires be kept 1 inch from surface wired over instead of only § inch 
as in dry places. 

There are two objections to the use of steel conduits in damp 
places, first f that all metal work is especially liable to corrosion even 
when well enameled or galvanized; and second and more important , 



Fig. 95. Corroded Rosette Improperly Mounted on Damp Ceiling 


that water is apt to collect in the pipes and gradually deteriorate the 
insulation. This water results from moisture condensed from the 
air during changes in the temperature and amount of water vapor 
present in the atmosphere. This condensation is often sufficient to 
be very troublesome and often leads to the adoption of open wiring of 
special forms rather than a complete conduit installation. Usually 
conduit is used in the more exposed or crowded places only and in 
such parts of the plant as are less liable to dampness. 

In many breweries, packing houses, and other plants, walls and 
ceilings are continually dripping with water and in some factories 
fumes and vapors are present in large quantities at all times. A 









UNDERWRITERS’ REQUIREMENTS 


99 



fairly good solution of the problem 
is possible where only water is to 
be guarded against, but where cor¬ 
rosive vapors exist, no thoroughly 
satisfactory method has been de¬ 
vised to resist indefinitely the cor¬ 
rosive actions. A good asphaltum 
paint will protect cabinets and con¬ 
duits for a time and frequent re¬ 
painting will extend the life of 
these parts of the equipment for a 
considerable period. It is evident 
that current-carrying metal parts 
should be enclosed in tight boxes 
wherever feasible and very frequent 
and thorough reinspections of the 
entire equipment should be made 
followed by renewals as faults de¬ 
velop. Fig. 95 shows the corrosion 
on a rosette improperly mounted. 

In rooms where dampness is 
excessive the wires are sometimes 
run open in inverted wood troughs, 
one form of which is shown in Fig. 
96 and in detail in Fig. 97. This 
trough serves to separate the wire¬ 
way entirely from a wet ceiling and 
the sloping surfaces serve to carry 
the moisture away from the knobs 
on which the wires are held. The 
V-shaped blocks are spaced about 
feet apart and care is taken to 
make a close joint where the run¬ 
ning boards come together at the 
top. The whole is thoroughly 


painted with an insulating paint. Fig. 96. Inverted Troughs on Damp 

Fig. 98 shows open wiring on flat 

running boards for moderately damp places like basements or cellars. 








100 


UNDERWRITERS’ REQUIREMENTS 



Fig. 97. Section of Wiring Trough 


I ^ THICK 



Fig. 98. Example of Wiring on Running Boards in Damp Place 



















UNDERWRITERS’ REQUIREMENTS 


101 


All drop cords should be of extra heavy reinforced type or of 
standard rubber-covered wire and only weatherproof keyless sockets 
should be used with all wire joints soldered, taped and painted in 
the best manner. Motors and their resistance boxes or starters 
should be kept out of damp rooms if possible but if in such rooms, 
they should be installed with special reference to accessibility, 
cleanliness, and separation from wet floors or walls. Posts in middle 
spaces of rooms will generally afford better locations than side walls, 
which are always wet. Wood cabinets lined with slate or with stiff 
asbestos board are preferable in many wet places to metal enclosures 
if they are kept well painted inside and out with an asphalt or in¬ 
sulating paint. A single incandescent lamp in such cabinets if kept 
constantly burning will tend to keep the interior of the box dry, if 
the box is tight and if the door is kept closed. Such cabinets may 
well have a glass panel in the door to show the lamp and incidentally 
to mark the location when the room is dark. 

Wires in Molding. Wood Molding. Wood molding is one of 
the commonest forms of protection for wiring and when properly 
used affords a cheap and fairly satisfactory installation. From one 
point of view it seems inconsistent to take every precaution in other 
forms of wiring to keep wires away from direct contact with wood 
surfaces, such as ceilings and walls and hidden spaces in frame par¬ 
titions and floors, and on the other hand to permit them to be run 
in small grooves in strips of wood, as in molding work. The solution 
to this somewhat theoretical objection is found in the fact that in 
molding, the wires are completely enclosed in a wire-way especially 
designed for them, rather than allowing them to hang or be drawn 
over wood objects in a more or less accidental manner with no real 
protection, and a still better solution is to be found in the unques¬ 
tionable fact that experience has proven the use of molding under 
proper conditions to be satisfactory. 

Wood molding should never be used in concealed spaces or in 
damp places or for voltages over 300. The rule as to damp places 
precludes its use in cellars and basements, anywhere out-of-doors 
and generally on outside brick walls which are often more or less 
damp and likely to “sweat” and thus introduce moisture back of the 
molding. If the wood molding becomes soaked with water there 
is a liability of leakage of current from one conductor to the other or 


102 


UNDERWRITERS’ REQUIREMENTS 


71 , 

d cD 
0? J_ 


to “ground.” If conductors in molding become overheated by 
excess current the wood may become charred, and charred wood is a 
fair conductor. The possibility of fire from such a cause is evident. 

Molding should be made of hard 
c , C| Cq % ' wood and should be thoroughly 
impregnated inside and out with 
a paint- or moisture-repellant. It 
must be made in two pieces, a 
backing containing the wire 
grooves, and a capping. The 
tongue between wires must be at least \ inch thick and the wood 
under the grooves must be at least | inch thick. Figs. 99 and 100 

give the forms of two- 


Ac 


—Ab- 

- Aa* 

*- Ab- 

k ' 


s_ ) 


Fig. 99. Section of Two-Wire Molding 


,Ca- 


- Ac 


~Ab -j^Aa*|»-Ab^j* Aa Ab ~j*Ac 


r 

cQ 

L 


Fig. 100. Section of Three-Wire Molding 


and three-wire moldings. 
Larger moldings are 
sometimes used for 
heavier wires but gener¬ 
ally only small conduc¬ 
tors are placed in 
wood moldings. Only good rubber-covered wire should be used 
and no joints or splices made in the wires in the molding, but where 
branch taps are necessary some form of fitting approved for the 

purpose should be employed. Such 
fittings as the one shown in Fig. 
101 provide porcelain bases with 
suitable binding screws for the main 
and the branch wires and a cover 
over the joints. 

A large variety of receptacles 
(both for lamps and plug connec¬ 
tors), rosettes, etc., are available for 
use with wood molding and should 
be used instead of the ordinary pat¬ 
terns. Where it is desired to insert 
a snap switch, either a special switch 
approved for mounting directly on 
the molding, or else a sub-base of porcelain or hard wood on which 
the switch can be securely fastened should be employed. 







































UNDERWRITERS’ REQUIREMENTS 


103 


Wood molding is often used in connection with other types of 
wiring and in such cases special attention should be given to making 
a good mechanical job where the conductors enter or leave the mold¬ 
ing. Fig. 102 shows one form of protector for use in protecting 
wood molding at floor levels. The capping should be carefully 
and tightly nailed in place and under no circumstances should fix¬ 
tures be attached to molding or any hooks or nails be driven into it 
for the support of lamp cords or other objects*. The use of wood 
molding in show windows is undesirable: first, because of the damp¬ 
ness apt to exist there; and second , because in the process of decorat¬ 
ing windows and arranging displays of merchandise nails will surely 
be driven into the molding with resultant injury to the insulation of 
the wires. 

In conclusion, it may 
be said that wood-molding 
work is cheap and may be 
used properly, but is in¬ 
ferior to most types of wir¬ 
ing. 

Metal Molding. Re¬ 
cently several types of metal 
molding have been intro¬ 
duced which are free from 
some of the objections ad¬ 
hering to wood molding, 
and make possible a neat, inexpensive, and convenient installation. 
In these moldings, as in those of wood, the wire is laid in, not drawn 
in as in conduits, and is covered by a metal capping. At present the 
underwriters’ rules limit its use to circuits requiring not more than 
660 watts of energy. 

Special fittings are provided for angles, bends, taps, and crosses, 
and for switch, rosette, and receptacle bases. The molding can be 
bent for slight curves or offsets and when carefully installed gives 
good results. Fig. 103 shows such a molding and fittings. 

Metal molding must be continuous from outlet to. outlet and 
where it passes through floors it must be enclosed in an iron pipe for 
added protection. The backing must be secured by screws or bolts 
with heads countersunk so as not to obstruct the wire-way. Between 



Fig. 102. Molding Protection at Floor Level 
































104 


UNDERWRITERS’ REQUIREMENTS 


lengths of molding and at all fittings and outlet boxes the joints must 
be mechanically and electrically secured. The fundamental idea 
is to secure an absolutely continuous metallic conductor throughout 
the entire run of molding and in addition the molding must be well 
grounded in a permanent manner. In this respect the molding is 
regarded in the same light as rigid metallic conduit. The reasons 




Fig. 103. Metal Molding and Fittings 


for requiring good electrical continuity and grounding will be ex¬ 
plained later when the subject of conduit is considered. 

Concealed Work. This kind of wiring is also often called “knob 
and tube” work since the wires are held on knobs and passed through 
tubes of porcelain. The great advantage of this method of wiring 
consists in the cheapness and ease with which a building, especially 
a frame building, can be wired. The greatest objection to the knob 
and tube work lies in the fact that the wires are wholly unprotected 
from mechanical injury and the building is not protected from the 
results of arcing between wires in case of a cross or short-circuit. 





UNDERWRITERS’ REQUIREMENTS 


105 


It is no doubt true that knob and tube wiring is the least reliable 
form of electric work and inferior to good conduit or armored cable. 
Where wires can be run open and are not exposed too much to 
mechanical injury, they are probably somewhat safer than when 
concealed on porcelain supports in walls, floors, and partitions. Open 
wiring is not possible in residences or where good appearance is a 
requisite, and the somewhat greater cost sometimes prevents the 
use of conduit. Therefore, recourse is made to concealed wiring and 
where an installation is carefully 
made a reasonably good result 
may be obtained. In many cities 
concealed work is entirely for¬ 
bidden within “fire limits”, that 
is, in the closely-built sections, 
but it is still very extensively 
employed in places where first 
cost and quickness of installation 
are the prime factors. Only 
good rubber-covered wire is al¬ 
lowable in concealed work. 

Approved Installation in a 
Residence . The wiring of a resi¬ 
dence may be taken as an 
illustration of how the work 
should be done. The service 
wires are brought in through 
the wall near the ground pre¬ 
ferably in iron conduit' but allowably through bushings with 
drip loops outside, Fig. 104. As near as practicable to the point of 
entrance is placed the main service fuse and switch. These should 
be in a suitable cabinet though this is not obligatory. It should be 
understood that the wiring is placed during the erection of the 
building. In an ordinary frame building the wiring will be done 
just after the rough flooring and the partition studding has been 
placed, but before the lathing or any plastering is done. It should 
all be completed except the final connection of service wires, fixtures 
and fittings before any of it is enclosed or hidden, so that inspection 
may be made while all parts are accessible and visible. 



Fig. 104. Approved Wiring System 
on Entering Building 














106 


UNDERWRITERS' REQUIREMENTS 


From the service cabinet and meter as many circuits are run as 
may be required to feed the lamps and other devices to be connected, 



each being fused at the service center unless there is no change at that 
point in the size of wires. These circuits will pass up through the 
floor within porcelain tubes and will thence be carried on porcelain 
knobs fastened to the timbers and studding not more than 4J feet 
apart. Where the wires pass through floor timbers, porcelain tubes, 
straight and smooth, must be used in holes bored in the wood and 
just large enough for the tubes. In general we may say that knobs 

are used where the wires 
run parallel with floor 
beams, and tubes where 
the wires are run at right 
angles to the beams. 
Fig. 105 shows the gen¬ 
eral method and Fig. 106 
illustrates a two-piece 
knob of common type. 
These are preferable to one-piece knobs with which a tie-wire is 
necessary. The wires should preferably be run singly on separate 
timbers or studding, and must, except as noted below, be kept 




Fig. 106. Two-Piece Knob for Holding Wire 

































UNDERWRITERS’ REQUIREMENTS 


107 


everywhere 5 inches apart. Fig. 107 shows clearly the use of the 
knobs and tubes, but the tubing shown on two of the upper wires 



Fig. 107. Knob and Tube Wiring Showing Objectionable Features 


is objectionable and knobs should have been used instead. It is 
very desirable to use metal outlet boxes at all outlets and the added 
expense of so doing is not large. Where flush switches or receptacles 



108 


UNDERWRITERS’ REQUIREMENTS 



Fig. 108. Proper Use of Outlet Boxes 


Fig. 109. Wiring in Partitions Showing Use of 
Porcelain Tubes 

































UNDERWRITERS’ REQUIREMENTS 


109 


are used, metal boxes are absolutely necessary and in Fig. 108 several 
such boxes are shown as well as the method of mounting them on 
cross strips between the uprights. This illustration shows also an 
allowable use of flexible non-metallic tubing on wires in concealed 
work where it is impracticable to maintain the 5-inch separation. 

Fig. 109 shows at the bottom the extra porcelain tube which 
should be put on each wire passing through timber at the bottom of 
a plastered wall to protect it from the droppings of wet plaster which 
will fall on it during the process of closing in the wall surface. This 
picture also shows very clearly the flexible tubing which must sepa¬ 
rately enclose each wire at every outlet, reaching from the last porce¬ 
lain support into a switch box, or on ceiling or wall outlets, where 
no box is used, at least 1 inch beyond the surface. In the case of 
combination gas and electric fixtures the tube must extend at least 
flush with the outer end of the gas pipe as in Fig. 110. 

When in a concealed knob and tube system, it is impracticable 
to place the whole of a circuit on non-combustible support of glass 



Fig. 110. Section Through Floor Showing Use of Tubing or Loom 


or porcelain, that portion of the circuit which cannot be so supported 
must be installed with approved metal conduit, or approved armored 
cable, except that if the difference of potential between the wires is 
not over 300 volts, and if the wires are not exposed to moisture, they 
may be fished if separately encased in approved flexible tubing, extend¬ 
ing in continuous lengths from porcelain support to porcelain sup¬ 
port, from porcelain support to outlet, or from outlet to outlet. 

There can, of course, he no assurance that such fished wires do not 
lie in close contact with gas or water pipes, or other wires, and so there 
is need of the protecting tubing. 

In judging an installation of wires in concealed work, special 
attention should be paid to the wire joints. The rules allow these 
to be made in concealed spaces by means of soldered joints well 
covered with both rubber and friction tape. The wiring is usually 









110 


UNDERWRITERS’ REQUIREMENTS 


on the so-called “tap plan,” that is, taps or branches taken off wher¬ 
ever convenient. The necessity for good workmanship is evident 
as bad joints may easily set fire in dry floor and wall spaces. Fig. 



Fig. 111. Defective Joint Found in Actual Use 


111 shows such a defective joint which was found behind a lath-and- 
plaster partition. The joint was not taped and was hot when dis¬ 
covered. What is called the “loop-plan” is occasionally employed, 
all joints or taps being made at outlets where suitable boxes are 





UNDERWRITERS’ REQUIREMENTS 


111 


provided. This involves the use of more wire, makes the conditions 
at outlets more crowded, and is perhaps not as good for concealed 
work under the conditions usually obtainable. Either system is 
permitted by the rules. 

Armored Cable. Armored cable for interior wiring consists of 
double-braided, rubber-insulated, tw T in wires, covered with a spiral 
steel strip armor which protects the conductors from injury and is 
at the same time flexible. It is largely used in wiring old buildings 
since it can be drawn into concealed spaces without fear of injuring 
the conductors. It is also used for new work and is in some ways 
easier and cheaper than rigid conduit. Fig. 112 shows a piece of 
twin-conductor house cable and some of the fittings for use with it. 




Fig. 112. Armored Cable and Fittings 


With armored cable all joints must be made at outlets or in cabinets 
or junction boxes which are always accessible. No taps or joints 
are permissible except at such outlets or boxes. Great care should 
be taken to secure the armor very firmly to all outlet boxes in a way 
to give a good connection both electrically and mechanically, and 
the armor system must be permanently and reliably grounded. Where 
dampness may be expected there must be a lead sheath between the 
braided wires and the outer armor since the cable is not thoroughly 
moisture-proof. 

Armored cable installations are superior to molding, open work, 
or concealed work, in that the wires are better protected. The fact 
that twin wire is used instead of separate conductors is at least a 
theoretical disadvantage, and the cable unless leaded is not absolutely 
waterproof. 




112 


UNDERWRITERS’ REQUIREMENTS 


The chief disadvantages of armored cable as compared with 
rigid conduit, consist in the somewhat greater difficulty of making 
good connections to the armor at outlets and still more in the im¬ 
possibility of drawing out wires which have proved defective. How¬ 
ever, for many places a thoroughly good job can be done with armored 
cable. Fig. 113 shows a characteristic piece of work of this sort. 

Conduit Work. The earlier forms of conduit for interior wiring 
were made of paper or fiber and later of paper with a thin brass 
casing but these forms are now obsolete and entirely displaced by 
steel conduits made of either rigid pipe or flexible steel spirals. A 
limited use is still made of a form of conduit having a steel pipe 



Fig. 113. Wiring Showing Use of Armored Cable 


lined with a heavy paper impregnated with some material to exclude 
moisture. This is, however, going out of use and almost all conduit 
work is now done with unlined pipe. Conduit, however, differs from 
ordinary commercial pipe such as is used for gas, water, or steam, 
in that it is carefully cleaned in the process of manufacture and then 
protected from rust both inside and outside by a good baked enamel 
or by some form of zinc coating. Rigid conduit, Fig. 114, gives a 
rather more workmanlike job than flexible conduit, Fig. 115, but the 
latter can be used in some places, especially in old buildings where 


















UNDERWRITERS’ REQUIREMENTS 


113 


the use of rigid pipe would be impossible. If well fastened at all 
outlets and at all bends it affords a protection to wires second only 
to the rigid conduit. The complete protection afforded by conduit 



Fig. 114. Rigid Conduit 

both of the wires from mechanical injury, and of the surrounding 
parts of the building from fire resulting from a burn-out of the wires, 
makes conduit work undoubtedly the safest form of electrical in¬ 
stallation and one which is becoming more and more used, not only in 
the more expensive type of buildings but also in cheaper work as well. 

The essential principle of conduit work is to furnish a complete, 
strong and unbroken metal enclosure for. conductors between outlets 
and to give to this metal-pipe system an electrical continuity and 
carrying capacity sufficient to serve as a safe path for any current 
which the failure of conductors within may impose upon it, for a 
time long enough to operate the fuses or circuit breakers protecting 
such conductors. It is also an essential characteristic of a correct 
conduit installation that all conductors can be drawn in after the 
entire conduit system is put into position and can at any subsequent 
time be drawn oui in case any wire fails and must be replaced. 
These fundamental ideas will explain the reasons for most of the 
details prescribed in underwriters’ rules for conduit work. 

The construction details of 
steel conduit will be briefly 
discussed later and we consider 
here only methods of installing it. 

There must be no breaks in 
the conduit system, that is, the 
pipe must be continuous from 
outlet to outlet or junction 

boxes. At every outlet metal pig . 115 Flexible Condui 

boxes must be provided which 

the conduits must properly enter and to which they must be me¬ 
chanically secured. This is usually accomplished by means of either 







114 


UNDERWRITERS’ REQUIREMENTS 


threaded lock nuts on the pipe outside the box and threaded bushings 
on the pipe inside the box or by threading the conduit into tapped 
holes in the box. With flexible conduit a special approved clamp must 
be used for this purpose. Fig. 116 shows a box with a rigid conduit 
on one side and a flexible conduit on the other side together with a 
few of the fittings used in connection with it. No conduit having 
an internal diameter less than f inch is allowable since this is as 
small as will permit wires of required minimum size to be drawn in 


without injury. 

All elbows and bends in the piping must be so made that the 
conduit will not be injured and there should not be more than the 
equivalent of four quarter turns from outlet to outlet, not counting 
bends at the outlets themselves. If more turns are required, or 

wherever it would be difficult 
to draw in the wires, addi¬ 
tional outlets called junc¬ 
tion- or pull-boxes should be 
put in to facilitate the inser¬ 
tion of the wires. There 
should be no sharp edges, 
burrs or other obstructions 
either in conduits, at coup¬ 
lings between lengths, or at 
outlets, as they are apt to 
injure the wire coverings. 
Therefore, all ends of pipe 
should be reamed out before 
they are put into the pipe 
couplings or into boxes, and 
all bushings should have smoothly rounded edges. 

The entire conduit system must be installed complete and, in 
fact, all the mechanical work on the building must be completed as 
far as possible, before any conductors are drawn into the conduits. 

Pains must be taken to make all joints tight, so that there will 
be no bad electrical connections between parts of the metal system. 
It is not, of course, expected that under normal conditions the con¬ 
duits will carry any current, but under some circumstances if the 
insulation of a wire fails, current may pass over the conduit. If the 



Fig. 116. Outlet Box with Connecting Conduit 







UNDERWRITERS’ REQUIREMENTS 


115 


conduit is well grounded and all joints are well made a safe path to 
“ground” is afforded for a current large enough to blow the fuses 
protecting the circuit, thus cutting off the current before the pipe is 
overheated or burned through at the point where the bared wire has 
come into contact with it. 

Conduits and gas pipes must be securely fastened in metal 
outlet boxes so as to secure good electrical connection. Where boxes 
used for centers of distribution do not afford good electrical connec¬ 
tion, the conduits must be joined around them by suitable bond wires. 
Where sections of metal conduit are installed without being fas¬ 
tened to the metal structure of buildings or grounded metal piping, 
they must be bonded to a permanent and efficient ground con¬ 
nection. 

It is rarely possible to perfectly insulate a conduit system 
throughout, and a positive ground is, therefore, required, so as to pro¬ 
vide a definite path for leaking currents and thus prevent them from 
escaping through parts of a building, etc., where they might do harm. 

The size of conduit which should be used for different sizes of 
wire or for different numbers of wires of specified size, depends upon 
the length of run between outlets where wires can be pulled or fed in, 
upon the number and the radius of bends and the thickness of the 
insulation on the wires. The rules, however, state that the same 
conduit must not contain more than four two-wire, or three three-wire 
circuits of the same system, except by special permission of the 
Inspection Department having jurisdiction, and must never contain 
circuits of different systems, that is, from different generators 
whether of the same voltage or not or whether both d. c. or a. c. or 
one of each. 

In tall buildings special provision must be made to support the 
conductors in the vertical conduits to remove their weight from the 
connections, and the spacing of supports in such cases is prescribed as 
follows: No. 14 to 0 every 100 feet; No. 00 to 0000 every 80 feet; 
0000 to 350,000 c. m. every 60 feet; 350,000 c. m. to 500,000 c. m. 
every 50 feet; 500,000 c. m. to 750,000 c. m. every 40 feet; 750,000 c. 
m. every 35 feet. 

The following methods of supporting cables are recommended: 

(1) A turn of 90 degrees in the conduit system will constitute a satis¬ 
factory support. 


116 


UNDERWRITERS’ REQUIREMENTS 


(2) Junction boxes may be inserted in the conduit system at the re¬ 
quired intervals, in which insulating supports of approved type must be in¬ 
stalled and secured in a satisfactory manner so as to withstand the weight of 
the conductors attached thereto, the boxes to be provided with proper covers. 

(3) Cables may be supported in approved junction boxes on two or 
more insulating supports so placed that the conductors will be deflected at 



Fig. 117. Method of Supporting Conduit Risers 

an angle of not less than 90 degrees, and carried a distance of not less than 
twice the diameter of the cable from its vertical position. Cables so suspended 
may be additionallv secured to these insulators by tie wires. 

The second method is illustrated in Fig. 117 where a space has 
been left in conduit risers. The conductors will be held to the back 

























UNDERWRITERS’ REQUIREMENTS 


117 


plates by clamps and the whole will finally be enclosed in a heavy 
steel box which will form the connecting bond in the riser system. 
Fig. 118 shows one form of cable clamp used for this purpose. 

Wires for conduit must be rubber- 
covered and have a double braid over 
the rubber. Twin wires are universally 
used for the smaller sizes, No. 14 to No. 

10, and these have a braid over the rub¬ 
ber of each conductor and a second braid 
over both conductors together. For 
larger sizes single conductors, double 
braided, are used and for the largest 
sizes only one conductor is usually run in 
a pipe It should be noted, however, 
that for alternating-current systems the two or more wires of any 
one circuit must be drawn in the same pipe since otherwise there 
will be excessive heating of the metal pipes due to a magnetic action 
peculiar to alternating currents and known as “induction.” Fig. 
119 shows three large feeder ducts passing out of a steel service box. 
In this case the circuit was three-wire direct current. 

The design of a conduit system of wiring for a large building 
may be a problem of some magnitude involving no little engineering 
skill and experience if the most economical, efficient, and sightly 
results are to be secured. A full exposition of the methods followed 
and the reasons for them does not fall within the scope of this book 
but the following general considerations are of some value in judg¬ 
ing both the advantages of conduit work and the excellence of any 
given installation. In all conduit work it should be remembered 
that no taps or joints are permitted in conductors except at outlets, 
junction boxes, and cabinets, and that all these must be in accessible 
places. By accessible is meant accessible not only during the process 
of installation of the system but also after the building is completed 
and in use. A junction box is not “accessible” if it is necessary in 
order to get at it, to take up portions of floors or make openings in 
ceilings or side walls. This prime requisite of conduit work makes 
it necessary to run all conductors on the “loop” plan which means 
that all lines, branches, and control circuits (as to switches) will con¬ 
sist of a pair of wires starting at the same point at some outlet box 



Fig. 118. Cable Clamp Used in 
Conduit System 
















118 


UNDERWRITERS’ REQUIREMENTS 


or cabinet and running generally in the same pipe to some other out¬ 
let where the socket, switch, or other device is placed. Conduit 
systems may be made with the piping concealed or exposed as may 
be dictated by the character of the building and the finish desired. 

In frame buildings where the electric installation is put in during 
the original construction the conduits are placed after the main 
framing and partitions are in position and will be arranged to pass 
through walls and floors with the outlet boxes and cabinets set and 
fastened securely at or very near the places where they will be when 



Fig. 119. Steel Service Box and Feeder Pipes 


the building is done. In laying out such work little regard need be 
paid to having pipes run straight or parallel to each other so long as 
they are so shaped at bends and otherwise disposed as to make it 
easy to draw in the wires. The chief considerations are the position 
of outlets, the proper pipe sizes for the wires, and the proper subdi¬ 
vision to secure correct fusing of incandescent lamp circuits accord¬ 
ing to the rule which permits not over 660 watts on any such circuit. 





UNDERWRITERS’ REQUIREMENTS 


119 


Characteristic Conduit Installation. Fig. 120 shows a char¬ 
acteristic layout for conduits and outlets for a small apartment. 



Fig. 120. Characteristic Layout for Conduits and Outlets in Small Apartment 



At the point marked “riser” a main conduit rises from the “service” 
in the basement and on each floor has a cabinet where the supply 

wires for that floor are taken 
off and fused and where the 
meter may be installed. In 
laying out such a system of 
conduits or in inspecting it 
one should start from the 
service entrance cabinets 
from which the conduits 
should enter. If this cabi¬ 
net contains not only the 
service fuses but also the 
sub-fuses of the branches, 
the several circuits should be 
examined to see that they are 
properly fused, and that they 
are distributed in pipes of 
adequate size. The several 
runs of conduit should then 
be followed throughout their 
entire length to see that all 
couplings between pipes and 
all attachments to outlet 


SI 


Fig. 121. Approved Ceiling Support for Conduits 

boxes are thoroughly tight and that both conduits and boxes are 





































































120 


UNDERWRITERS’ REQUIREMENTS 


fastened by straps or screws so that no strain will be put upon the 
screwed joints. Care should be taken to see that there are no sharp 
bends anywhere and that no more outlets are set in one circuit than 
is proper and that all are accessible. Observe whether due provision 
has been made for installing fuses wherever there will be a change in 
the size of wire. It is often cheaper and more satisfactory to continue 
a somewhat larger wire than is required, than to change to a smaller 
size and insert fuses. In no case should fuses be installed in small 
outlet boxes, but only in junction boxes or cabinets of ample size. 

After the wires have been drawn in, observe whether it is pos- . 
sible to trace a given pair from outlet to outlet and if there is doubt 



Fig. 122. Fittings Used for Exposed Conduit Work 


on any point it may be necessary to demand that certain wires be 
drawn out for inspection. There should, of course, be absolutely 
no corners, Y-shaped fittings, or other devices in any place which 
will prevent any or all wires from being taken out and replaced at 
any time even after the building is completed. It is well to observe 
whether there are bends or traps in the piping in which water result¬ 
ing from condensation or otherwise can collect. All conduit should 
be practically uninjured by the work done on it, all pipe ends should 
be threaded and reamed and the entire metal system should be firmly 
supported and secured so as to be free from vibration or rattle. The 
system must be electrically continuous throughout and must be 
permanently and effectively grounded. 







UNDERWRITERS’ REQUIREMENTS 


121 


Where conduits are run exposed, more attention should be paid 
to appearances. All runs should be in straight lines parallel to walls 
of the rooms and arranged in a symmetrical and orderly manner. 
Smaller sizes of conduit can usually be fastened to ceilings or walls 
by ordinary pipe straps, but where large conduit is used some form 
of pipe hanger is required and there are special patterns of a variety 



Fig. 123. Conduit Wiring Circuits Laid before Partitions and Floors 
are Completed 

of designs. Where a number of conduits are run parallel the form 
of supports shown in Fig. 121 is excellent. With exposed conduit a 
variety of fittings can be used for outlets, junction boxes, and the 
like, which add much to the neat appearance and also provide for 
joints and traps as well as for mounting sockets, switches, and other 
small devices. Fig. 122 shows a few such fittings for exposed conduit. 




























122 


UNDERWRITERS’ REQUIREMENTS 



These are made with threaded nipples into which the conduit may 
be screwed and when so used these fit¬ 
tings need not be fastened to the wall 
or ceiling in any way, but are considered 
to be firmly enough held by the pipe 
itself. However, if boxes requiring lock 
nuts and screw bushings on pipes are 
employed they must be separately se¬ 
cured to the surface of wall or ceiling 
by screws and all cabinets of any size 
whatever should be so fastened. Ex¬ 
posed conduit work is usually employed 
in factories, warehouses and elsewhere 
if the finish of room does not require all 
piping to be out of sight. 

Some special problems arise in con¬ 
duit work in fireproof buildings. In 
these types of structure the conduit 
work is done after the rough floor work 
is completed, the conduits being usually 
laid on the tile floors so as to be covered 
by the top finish of cement usually laid 
over the tiling. Fig. 123 shows such 
an installation and it will be noted 
that the baseboard and all side-wall 
outlets have been placed, before the 
tile partitions between the rooms, have 
been set. In a fireproof building it will 
generally be necessary to run the main 
risers before the floors are made and to 
arrange for the distributing cabinets on 
each floor. Fig. 124 shows a cabinet 
in place in the unfinished partition 
with two conduit risers entering it from 
below and the branch circuits going 
out from the top to various parts of the 
building. The cabinet in this case has 
Fig. i 24 . wire Distributing Cabinet been fastened by rods above and below. 











UNDERWRITERS’ REQUIREMENTS 


123 


In concrete buildings it is often somewhat difficult to place 
the conduits and the boxes so as to clear the reinforcing material 
and at the same time keep the metal parts well covered at all points. 
Special methods have to be adopted varying with the type of con¬ 
crete construction employed but in all cases an effort should be made 
to secure proper draining of conduits. The water from the concrete 



Fig. 125. Method of Placing Outlet Boxes in Tile Partitions 


is very liable to get into the conduit system at the outlets or else¬ 
where, and when once this occurs it is difficult to get it out. Its 
presence is, however, most objectionable as it tends to injure the 
piping by rusting and also to deteriorate seriously the insulation 
coverings on the wires. This is a difficulty which should be considered 
in locating outlets in such a building and which requires very great 
care to entirely overcome. It is a matter of some difficulty at times 








124 


UNDERWRITERS’ REQUIREMENTS 


to secure outlet boxes on the concrete forms or in correct position 
where tile is to be used so as to be sure that they will be in the right 
position and firmly held when the building is completed. Ceiling 
boxes should be held by special hangers from each box up into the 
tile or concrete and reliance should not be placed on the conduits 
alone to hold them. Boxes set where tile partitions are to be sub¬ 
sequently erected should preferably be held by iron straps to adjacent 
steel work wherever this is practicable. Fig. 125 shows how this 
may be done. 


SPECIAL INSTALLATIONS 
DECORATIVE AND COMMERCIAL LIGHTING 

Decorative Lighting. Decorative lighting by means of incan¬ 
descent lamps is often desired either inside or outside buildings. The 
chief hazards of such work lie in the use of inferior materials hastily 
put together and poorly located and fused. The voltage should never 
exceed 150 volts in such work and not more than 1,320 watts of 
energy should ever be allowed to be dependent for protection upon 
a single cut-out. It is highly improper to take current for such sys¬ 
tems from ordinary outlets which are wired for only small currents, 
since by so doing the wires to the outlets will be overloaded and the 
proper fuses for the wires will have to be replaced by others of too 
great capacity to furnish safe protection. The supply should be 
taken only from points on the circuit (as at distribution boards or 
panels) where the correct fusing and wiring can be provided for. 

There is obviously no real distinction between decorative lighting 
and ordinary lighting arrangements except that the former is usually 
put up for temporary use only. If these temporary circuits are in¬ 
stalled in a workmanlike manner and if good wire and good sockets 
are used there need be no special hazard provided the wires are not 
tacked up in a manner liable to injure the insulations and are not 
made to serve as supports for inflammable decorations of cloth or 
paper. Festoons of keyless weatherproof sockets well connected to 
standard rubber-covered wire (not flexible cord), and kept separated 
. from all combustible stuff, do not present any great hazard but no such 
installation should be made without great care and should never be 
allowed to remain as a permanent installation unless it is fully pro- 


UNDERWRITERS’ REQUIREMENTS 


125 


tected and made the equivalent of standard work in every respect. 
The use of extra “carrier” wires for such festoons is very desirable 
as they remove all strain from the current-carrying wires and con¬ 
nections. Show windows are very frequently decorated by tem¬ 
porary electric displays of all sorts especially during the holiday sea¬ 
son and such window displays are very often wired in utter disregard 
of all safe rules. The wires are often made to serve as supports for 
merchandise of very inflammable material, and where tinsel, cotton 
batting, and similar materials are employed, the conditions are ex¬ 
cellent for a rapidly-spreading fire if any electric failure should 
occur. Incandescent lamps are often allowed to lie against inflam¬ 
mable materials and small motor-operated advertising or other nov¬ 
elties may contribute their share to the danger. The fact that an 
installation of this sort is “temporary” does not lessen the hazard 
so long as it is in operation and should not in any sense be considered 
as an excuse for allowing practices which are known to be danger¬ 
ous. Property owmers are often ignorant of the extent to which 
ill-advised temporary decorations jeopardize their buildings. 

In show-w T indow lighting, the lamps, whether temporary or 
permanent, should be placed either in the front or the top of the 
window, proper reflectors being used to throw the light upon the 
goods displayed. This plan almost invariably gives better results 
in fighting than lamps placed in sight, and scattered through the 
window and among the goods, and is also very much safer. 

Outline Lighting. A very common form of decorative work con¬ 
sists of what is known as outline lighting. In this class of work rows 
of incandescent lamps are used on the exterior of buildings to outline 
the chief architectural features, to mark entrances, etc. Lamps 
and wdring used for such purposes inside buildings come under the 
regular rules for inside work but outside outline fighting may have 
some special characteristics. Only low potential circuits (under 550 
volts) should be used and the wiring either open work or in conduit. 
Molding, either wood or metal, is not allowable since it does not 
afford protection against moisture. In open work the same spacing 
of wires from the surface (1 inch) and from each other (2| inches) 
must be maintained as in all exposed wiring in damp places. If the 
use of flexible tubing is necessary, as at crosses or where regular 
spacing cannot be maintained, the ends of the tubing must be sealed 


126 


UNDERWRITERS’ REQUIREMENTS 


and painted so as to exclude moisture, and for a similar reason' 
armored cable, if used, must be of the type having a lead sheath over 
the conductors and under the armor. 

In order to assure proper control and fusing of outline-wiring 
circuits they should have their own separate switch and fuses enclosed 
in a suitable metal cabinet which must be watertight if placed out- 
of-doors. Small candle-power lamps (2 to 4) are almost always used 
for outline wiring and may be so grouped on the circuits that not 
more than 1,320 watts will be dependent on a single pair of fuses 
but in no case should more than 66 sockets or receptacles be con¬ 
nected to a single circuit. This limitation of the number of lamps 
is to prevent any trouble in a single socket causing too much arcing 
or burning before the fuse protecting its circuit will open. All the 
sockets and receptacles in outline wiring must be of the keyless 
porcelain type and the wires connecting them must be soldered to 
the lugs. With conduit work in outline wiring special attention 
should be given to making the entire system as water tight as pos¬ 
sible. The receptacles set in the covers of steel outlet boxes or 
conduits should be provided with rubber gaskets, and similar rubber 
rings may also be used to advantage about the bases of the lamps to 
prevent the entrance of water into the receptacles themselves. 

Electric Signs. An endless variety of electric signs are shown 
on the streets of cities and towns today, from the simplest illuminated 
panels to huge structures containing many hundreds of lamps and 
operated by very complicated “flashing” machines. An elaborate 
treatment of all types of signs cannot be given here. The large 
signs erected on roofs should come under the supervision of the 
building department of a city or under both electric and building 
departments. Many of these signs are hazardous because of the 
obstruction they offer to firemen, the large amount of power they 
use, and the inadequate provision made for cutting off current from 
them either automatically or by hand switches in accessible places. 
No combustible material should be permitted either in or near such 
signs. It is preferable that all wiring be in conduit but where this 
is impracticable, the supports and spacing of wires should be excellent 
and the sides of the circuits should be kept apart preferably by 
bunching all wires of one polarity into cables covered by waterproof 
and slow-burning jackets. The mass of inflammable rubber-covered 


UNDERWRITERS' REQUIREMENTS 


127 


and braided wire necessary for a large sign requires skilful arrange¬ 
ment to avoid a serious blaze in case of failure. 

Signs of every size should be “all metal,” sheet steel being not 
less than No. 28 U. S. gauge and frames being in all cases of ample 
stiffness and strength. Metal parts should be well protected by 
paint or enamel and the sign structure should be reasonably water 
tight but have drainage holes in the bottom to let out any water 
that may collect inside. The wires should be soldered to all recep¬ 
tacle terminals, should be only rubber-covered and should be brought 
out through the sign structure either in conduit or in well spaced 
porcelain bushings with drip loops. In supporting the sign attention 
should be paid to preventing wear or abrasion of the leading-in wires 
by swinging of the sign. Many cities have especially elaborate rules 
for the placing and supporting of signs designed to reduce the pos¬ 
sibility of their falling or of their interfering too much with the use 
of fire-escapes or the work of firemen in case of fire in the building. 

Sign Flashers. The flashing, and other very elaborate effects of 
electric signs, are accomplished by the use of what is known as sign- 



Fig. 126. Sign Flasher for Displaying Electric Signs 


machines or flashers, Fig. 126, which consist essentially of motor- 
driven drum switches often of great size and complexity. These 
machines are hazardous chiefly from their use of many necessarily 
rather frail single-pole brush switches with large numbers of connect¬ 
ing wires leading to them and from the fact that being essentially 
automatic, such machines are liable to be neglected and poorly 
maintained. The location of a sign machine should be such as to 
remove it from the chance of accidental injury or tampering, to 
permit the direct and well arranged running of circuits to it, and 
to render it accessible for cleaning, adjusting, and inspecting at all 
times. A flasher should never be installed in a closet or other con¬ 
cealed space. If it is impracticable to mount it outside the build¬ 
ing near the sign, as is preferable, a location should be chosen that 




128 


UNDERWRITERS’ REQUIREMENTS 


will reduce as far as possible the amount of wiring required. A plat¬ 
form or shelf over a doorway and just inside the building is often 
the best place available. A heavy metal cover must be provided 
but this should be removable so as to permit adjustment of all operat¬ 
ing parts within. 

THEATER WIRING 

General Specifications. The electric equipment of theaters is 
of great importance both because of the large value of these properties 
and also because of the life hazard involved in any public building 
in which large numbers of people assemble. The electric equipment 
of a modern theater of the first class is generally very elaborate, 
involving as it does not only the brilliant illumination of large rooms, 
the stage, and many smaller apartments, but also the power equip¬ 
ment for handling curtains, scensry, and other paraphernalia and 
the numerous devices of special character for producing stage effects. 
In the meaning of the Code a theater is defined as a building or 
part of a building in which it is designed to make a presentation 
of dramatic, operatic or other performances or shows for the enter¬ 
tainment of spectators, which is capable of seating at least 400 
persons, and which has a stage for such performances that can be 
used for scenery and other appliances. All theater wiring not 
specifically covered by special rules should conform to standard 
rules and requirements for work of its class. The special rules 
naturally divide into those which concern the protection of life 
chiefly and those which concern primarily the fire hazard, though, 
of course, the latter have a direct bearing on the life hazard also, 
since even a relatively small fire in a theater may produce a panic 
resulting in injury to many persons. 

For the purpose of insuring the most reliable maintenance of 
proper lighting and consequent safety and freedom from panic in the 
audience, the underwriters’ rules suggest that where the source of 
electric supply is outside the building there must be at least two 
separate and distinct services where practicable, fed from separate 
street mains, one service to be of sufficient capacity to supply current 
for the entire equipment of the theater, while the other service 
must be at least of sufficient capacity to supply current for all 
emergency lights. By “emergency lights” are meant exit lights 
and all lights in lobbies, stairways, corridors, and other portions of 


UNDERWRITERS’ REQUIREMENTS 


129 


the theater to which the public have access which are kept normally 
lighted during the performance. Where source of supply is an 
isolated plant within the same building, an auxiliary service of at 
least sufficient capacity to supply all emergency lights must be in¬ 
stalled from some outside source, or a suitable storage battery within 
the premises may be considered the equivalent of such service. 

The principal hazards in a theater result from the stage equip¬ 
ment since it is there rather than in the auditorium that large cur¬ 
rents are used; many conductors are required, and all sorts of special 
devices are employed often in close proximity to combustible material 
such as scenery. Furthermore, the constant changes, additions and 
special requirements for different productions result in subjecting 
the electric equipment to exceptionally hard wear and tear on theater 
stages so that proper upkeep can be secured only by the use of the 
most approved materials and methods and constant re-inspections. 
All permanent electric work on the stage side of the proscenium 
arch must be in conduit or armored cable except in border lights 
and on the stage switchboards. 

All of the stage circuits, footlights, borders, arc lamps, and 
usually all house lighting are controlled from a switchboard located 
on the stage at one side of the proscenium arch. This results in a 
very large number of cables and wires being concentrated at this 
board and calls for very careful planning if anything like safe and 
orderly arrangement is to be secured. The space available is often 
small and circuits are, therefore, liable to be unduly crowded and 
confused. The design of a large stage switchboard calls for no little 
engineering skill, which is too often noticeable chiefly from its entire 
absence especially in older houses. The space back of the board 
should be of ample size, readily accessible but entirely enclosed by 
absolutely fireproof walls and doors and preferably directly ven¬ 
tilated to the outside by a special brick chimney or large flue. All 
circuits should be controlled by substantial knife switches and fused 
by standard enclosed cut-outs in accessible places where their opera¬ 
tion will not in any way be liable to set fire to the wire insulations. 
It is very desirable that all wires on the back of the board be covered 
with asbestos jackets and be well spaced. 

Dimmers. The dimmers or special rheostats for gradually 
reducing the intensity of incandescent lamps either in the auditorium 


130 


UNDERWRITERS’ REQUIREMENTS 


or on the stage constitute often a very large installation of heat- 
developing apparatus which should be carefully located on iron 
brackets or on open galleries absolutely separated from any possible 
contact with burnable stuff. Fig. 127 shows a bank of dimmers. 

Footlights and Borders. Among the more important fixed- 
appliances are the footlights, border and strip lights, and the stage- 
floor pockets. The footlights should be installed either in conduit or 
on special steel boxes which entirely enclose the lamp receptacles. 
Border lights must be substantially made of heavy sheet metal (at 
least No. 20 metal gauge) and must be wired with slow-burning wire 
because they necessarily get very hot from the lamps they contain. 



Fig. 127, Bank of Theater Dimmers 


Footlights, on the other hand, do not get so hot and their location 
makes them susceptible to moisture during the cleaning or washing of 
the floor of the stage. They should, therefore, be wired with rub¬ 
ber-covered wire. 

Border lights must be raised and lowered and, therefore, current 
must be carried to them by means of cables composed of stranded 
rubber-covered wires. Such cables should be in rigid conduit to the 
point where the cable leaves for the border and at this point a suit¬ 
able junction box should be provided. The method of bringing the 
cable from this box and also of leading it into the border structure 
should receive special attention to prevent undue strain being brought 
to bear upon it at these connections. The cable should be of the 




UNDERWRITERS’ REQUIREMENTS 


131 


special type designed for this service. Figs. 128 and 129 show sec¬ 
tions of footlights and borders ready for installation on a stage. 



Fig. 128. Section of Theater Footlights 


Stage Pockets. Stage pockets (see Fig. 130) are iron boxes, 
having trap doors flush with the stage floor and containing receptacles 



Fig. 129. Section of Theater Borders 



for special plugs and cables by means of which arc lamps, flood lamps, 
and other temporary and portable devices may be connected. The 

circuits should have a capacity of 
at least 35 amperes for arc lamps 
or 15 amperes for incandescent 
lamps, wired to the full capacity 
and controlled from the stage 
board. They should contain no 
fuses and should be so constructed 
that accumulations of dust or rub¬ 
bish in them cannot interfere with 
their safe use or be liable to cause 


Fig. 130. Stage Pocket 


























132 


UNDERWRITERS’ REQUIREMENTS 


a fire from any arc resulting from withdrawing the plug. These 
pockets may be for one or several plugs. 

Portable equipments for use on theater stages include, in addition 
to special devices of all sorts—arc lamps, bunch lamps, strip lights, 



Fig. 131. Non-Approved Form of Stage 
Arc Lamp 


pin plug connectors, and portable 
plugging boxes. Fig. 131 shows a 
poor form of stage arc lamp of 
the type supposed to have caused 
the Iroquois theater fire in Chi¬ 
cago, while Fig. 132 shows a 



Fig. 132. Approved Form of Stage 
Arc Lamp 


modern approved form. These should be of approved pattern, 
made of non-combustible materials only and fitted with standard 
stage cable the specifications for which are as follows: To consist 
of not more than three flexible copper conductors each of capacity 
















UNDERWRITERS’ REQUIREMENTS 


133 


not exceeding No. 4 B, & S. gauge, built up of wires not smaller 
than No. 26 B. & S. gauge; each conductor to be covered with a 
cotton wind and with rubber insulation of thickness equivalent to 
standard rubber-covered wire and a saturated braid; the conductors 
to be twisted together with a filler (jute or a similar material) to 
make the cable round and to act as a cushion; the whole to be 
covered with two weatherproof braids. Such a cable will withstand 
considerable hard usage and has sufficient mechanical strength to 
endure the severe conditions of this service. 

Special Lighting Circuits and Stage Effects. The proper 
installation of all of the stage devices just mentioned is such an 
important feature of the safety of the theater that special rules are 
established for portable conductors, for lights on scenery, for fes¬ 
toons, and for the many special effects, such as lightning, etc. 

Requirements for Stage Auditoriums. The special requirements 
for the lighting of stage auditoriums are as follows: All wiring must 
be in either conduit, metal moldings, or armored cable; all fuses 
must be in enclosed cabinets; exit lights must not have more than 
one set of fuses between them and the main service fuses, and 
together with all hall and corridor lights, must be supplied inde¬ 
pendently of the stage lighting and be controlled only from the lobby 
or front of the house. All of these requirements are made to insure 
adequate lighting for the audience in case of a fire, to render the 
exit and corridor lights independent of any accident to circuits re¬ 
sulting from a stage fire, and to insure a certain amount of illumi¬ 
nation at all times in order to permit firemen to work to advantage 
in case of a fire in the auditorium or corridors leading thereto. 

MOVING PICTURE THEATERS AND MACHINES 

Interior Equipment. The very large number of moving-picture 
theaters which have come into use in the past few years has intro¬ 
duced a special hazard of considerable extent and a large number of 
disastrous fires in such installations have been recorded. See Fig. 
133. These theaters have often been established in rooms originally 
intended for stores and their equipment has been of the poorest 
character. This condition is now somewhat improved as the danger 
has become more generally recognized and has been made the sub- 


134 


UNDERWRITERS’ 


REQUIREMENTS 


ject of special legislation both by insurance interests and by municipal 
and town ordinances. 

Causes of Danger. The lighting of such theaters comes under 
the ordinary rules for electrical work but the peculiar danger lies 



Fig. 133. Showing Fire in Moving Picture Lantern Housing Due to 
Inadequate Precautions 


in the use of the arc lamp projection machines with the highly in¬ 
flammable “films” on which the pictures are printed. These films 






UNDERWRITERS’ REQUIREMENTS 


135 


are of cellulose coated with a chemical which burns with great 
intensity and rapidity and with an almost explosive force. A burn¬ 
ing film cannot be extinguished easily and evolves an enormous 
amount of heat and a dense and suffocating smoke. In displaying 
the moving pictures the film (usually 500 to 1,500 feet in length) is 
caused to pass rapidly in front of a special arc lamp taking from 25 
to 50 amperes at 110 volts, the light and also the heat of the arc being 
concentrated on a small area of the film about as large as a postage 
stamp by means of lenses set in the head of the machine. While 
the machine is in normal operation, the ribbon of film is moved so 
rapidly that no portion of it has time to become heated. If, however, 
for any cause, the advance of the film is checked so that a portion of 
it remains for even a few seconds exposed to the concentrated heat 
of the arc, it is at once ignited and the flame spreads to the adjacent 
parts of the film and the whole reel is liable to burn up with a very 
intense fire which may endanger the life of the operator and create 
a serious fire hazard to the building. The film may also be ignited 
by coming into contact with the hot lamp housing or with the resist¬ 
ance box which it is usually necessary to use in the arc lamp circuit 
to regulate the current. When alternating current is used special 
forms of transformers or coils may be used instead of the resistance 
box or rheostat and these are to be preferred since they do not get 
so hot. 

The arc lamp and rheostats should be constructed similarly to 
arc lamps and rheostats used on theater stages. Tight metal boxes 
made without solder must be used both to hold the reel from which 
the film is unwound in order to expose to view, and also to receive 
the film after it has passed through the machine head, and the open¬ 
ings in these boxes must be as small as possible, being regulated by 
rollers between which the film may pass or by shutters which can 
be instantly closed. The handle or crank used in operating the 
machine must be secured to the spindle or shaft, so that there will be 
no liability of its coming off and allowing the film to stop in front 
of the lamp. A shutter must be placed in front of the condenser, 
arranged so as to be readily closed. Extra films must be kept in a 
metal box with tight-fitting cover. Machines must be operated by 
hand and never by motor, since the failure of the motor, the slipping 
or breaking of a belt, or other like accident might stop the film. It 


136 


UNDERWRITERS’ REQUIREMENTS 


is essential that the film and the operation of the entire equipment 
receive the constant attention of the operator. The picture machine 
must be placed in an enclosure or house made of suitable fireproof 
material, must be thoroughly ventilated, and should be made large 
enough for the operator to walk freely on either side of or back of it. 
All openings into this booth must be arranged so as to be entirely 
closed by doors or shutters constructed of the same or equally good 
fire-resisting material as the booth itself. Doors or covers must be 
arranged so as to be held normally closed by spring hinges or equiv¬ 
alent devices. 

The construction, location, and wiring of this booth are the 
chief considerations in the equipment of a moving-picture theater 
and no pains should be spared to obtain the highest possible degree 
of security in every detail, in its construction and equipment, and 
the greatest care in its maintenance. 

CAR WIRING 

The electric wiring of cars requires some special rules which 
make certain exceptions to the general rules on the one hand and 
which make some special extra requirements on the other hand. 
These special rules have to do chiefly with the protection of car 
bodies and woodwork over all the electrical apparatus such as motors, 
resistances, contactors, and the like, and over such of the conductors 
as are not run in conduit. Other somewhat special requirements 
apply to wires, cables, and methods of making joints and connections 
in them, to the location and type of fuses and circuit breakers to be 
used, to special forms of conduit and wood molding, and to details 
of the lighting, heating, and air-pump circuits. Very carefully detailed 
requirements have been worked out for the main motor circuits 
and for the devices used in connection with them. As all of these 
requirements are of a very special character, the underwriters’ rules 
in this connection should be studied in detail by those directly 
interested in car wiring. 

Car houses or barns present some hazards which can be reduced 
by the observance of a few special rules which are determined chiefly 
by the fact that the railway circuit is normally grounded and thus 
requires somewhat different treatment from the ordinary under¬ 
ground light or power circuits. 


UNDERWRITERS’ REQUIREMENTS 137 

LIGHTING AND POWER FROM RAILWAY WIRES 

It should be especially noted that under no circumstances is it 
proper to take lighting or power circuits from trolley or third-rail 
railway circuits with a ground return. This, of course, does not and 
cannot apply to the electric railway cars, car houses, power, passenger 
and freight houses connected with the operation of electric railways 
in which the use of such circuits is obviously necessary and which 
can be specially guarded and supervised. For all other factories or 
buildings of all sorts, power from grounded railway circuits is for¬ 
bidden because of the danger of introducing into a building a circuit 
which has so much capacity back of it and which is thoroughly con¬ 
nected with the earth on one side. The inevitable fluctuation in 
voltage would also frequently require overfusing of the lighting 
circuits to prevent blowing fuses under normal conditions. 

HIGH= AND EXTRA HIGH=POTENTIAL SYSTEMS 

Classification. Under low, constant-potential systems are in¬ 
cluded all such as have voltages not over 550 volts but it should 
be noted that this distinction is an arbitrary one adopted for the 
classification of the rules on electrical work in relation to the fire 
hazard and is not made in just this way in classifying power-trans¬ 
mission lines and commercial systems from other viewpoints. In 
the underwriters’ rules any circuit attached to any machine, or 
combination of machines, which develops a difference of potential, 
between any two wires, of over 550 volts and less than 3,500 volts, 
shall be considered as a high-potential circuit and as coming under 
that class, unless an approved transforming device is used, which 
cuts the difference of potential down to 550 volts or less. Similarly 
if the difference of potential between any two wires is over 3,500 
volts the circuit is classed as an extra high-potential circuit unless 
an approved transforming device is used. This means that where 
the power is brought to a transformer on primary lines of not over 
3,500 volts and the secondary lines are not over 550 volts, the sec¬ 
ondary system of light or power is considered a low-potential system, 
but if the primary wires of 550 to 3,500 volts are direct to the power¬ 
consuming devices, as motors, without the outside transformer, 
the system is considered a high-potential system. If the primary 
lines are over 3,500 volts, the secondary lines beyond the transformers 


138 


UNDERWRITERS’ REQUIREMENTS 


are classed as high-potential and where no transformer is employed 
the circuit is extra high-potential. 

High- and extra high-potential systems are, of course, usually 
alternating-current circuits since only alternating current is ordi¬ 
narily used for over 600 volts. There are, however, a number of 
railway installations in the United States operating at 1,200 volts 
direct current and this type of power distribution is becoming some¬ 
what common. A 1,200-volt circuit is, of course, classed as high- 
potential. Use is made of high-potential circuits for both light and 
power purposes and its advantages over low voltages lie in the smaller 
sized copper that can be used to transmit a given amount of power 
over a certain distance with a specified percentage of loss due to the 
line wires. 

Requirements for Safety. For lighting, 2,200- to 2,500-volt 
circuits are used to transmit the power from the generating station 
to standard transformers at or near buildings where the light is used 
and these transformers step down the voltage either to 110 or 220 
volts or to a three-wire 110- to 220-volt system on the secondary 
side for the lamps. A very few cities and towns employ a 220- to 
440-volt three-wire secondary system but this is not usual or de¬ 
sirable in general, as most fittings, fuses, etc., are designed for not 
over 250 volts. 

Motors designed for alternating current may, of course, be 
operated on these secondaries and such secondary circuits whether 
for light or power are simply low-potential circuits provided the 
higher voltage lines end at transformers suitably installed outside 
the buildings or as near as possible to the point where the primary 
wires enter the buildings. The outside location is much to be pre¬ 
ferred. 

When circuits having high-potential transformers are located 
inside of buildings they should be placed in an enclosure made of 
fire-resisting material such as brick, tile, or concrete. The enclosure 
should be used for nothing but the transformers and should be kept 
locked. Fig. 134 shows such an enclosure or vault for transformers. 
It is a good plan to arrange the transformer room or enclosure so that 
it can be entered only from outdoors, since then, even if the door 
should happen to be open at the time of a fire in this room, it is 
probable that no especial harm would be done. Moreover, the fire 



UNDERWRITERS* REQUIREMENTS 139 

could doubtless be better handled from the outside. The trans¬ 
formers must be thoroughly insulated from the ground, or per¬ 
manently and effectually grounded, and the enclosure in which they 
are placed must be practically air-tight, except that it must be 


Fig. 134.* Approved Method of Protecting Transformers in a Vault 

thoroughly ventilated to the outdoor air, if possible through a chim¬ 
ney or flue. There should be at least six inches of air space on all 
sides of the transformer. This rule will permit of either the insulating 
or grounding of transformer cases as seems most advisable under the 
conditions, but will require that with either arrangement the work 

* Courtesy Inspection Department, Associated Factory Mutual Fire Insurance Companies, Boston, Mass. 


















140 


UNDERWRITERS’ REQUIREMENTS 


be well done, and that unless good insulation be provided the cases 
be definitely grounded. The object of an air-tight enclosure is to 
prevent smoke from escaping or fire from spreading, in case the 
transformer coils should become overheated from an overload or 
should be ignited by a break-down in the insulation between the 
primary and secondary coils. This is especially important with oil- 
cooled transformers in which the danger from an oil-fire is added to 
the usual electrical hazards involved. 

Wherever high-potential circuits are brought into a building 
only rubber-covered wire should be used and special care should be 
taken with the wire supports to protect the wires from mechanical 
injury since a failure resulting in an arc is very dangerous at this 
high voltage. Substantial boxing about wires on side walls and 
running boards where circuits cross floor timbers are very necessary, 
as, in fact, are all the precautions and rules for good wiring and good 
workmanship throughout. A very large number of motors are now 
used taking 2,200 to 2,500 volts at the motor terminals. Such power 
installations can be made reasonably safe by careful planning of the 
circuits and by excellence of installation and upkeep of all wiring 
and connected apparatus. No multiple-series or series-multiple 
system of lighting is allowed on high potential circuits. 

Extra high-potential circuits are not allowed either in or over 
buildings except power stations and substations. Where extra 
high-potential primary circuits (over 3,500 volts) supply transformers 
the secondary wiring must be installed as high-potential circuits 
(550 to 3,500 volts) unless the primaries are installed in complete 
compliance with the rules governing outside work on constant- 
potential pole lines over 5,000 volts or are wholly underground 
within city, town, and village limits. 

In concluding the consideration of these higher voltage cir¬ 
cuits, it is proper to refer again to the fact that the higher the volt¬ 
age employed the greater the need of care in instadation, upkeep, and 
operation since the results of accidental arcs are more serious. 

SIGNALING SYSTEMS 

Wiring Requirements. The wiring and other devices in the 
great variety of signal systems now employed in buildings, present 
as a general rule no hazard except their liability to become crossed 


UNDERWRITERS’ REQUIREMENTS 


141 


either outside or inside buildings with electric light, heat, or power 
circuits. Such signal systems include telephone, telegraph, district 
messenger and call bell, fire and burglar alarm, and all similar ap¬ 
pliances and circuits. It is seldom that the wires of any of these 
systems are installed in buildings with the same care as are those 
for lighting or power, and the insulations employed on signal wires 
are very generally far inferior to those specified for light and power 
circuits. Furthermore, since signal systems are usually operated 
from either primary or secondary (storage) batteries of low voltage 
and limited current capacity, they may be and commonly are in¬ 
stalled with little attention to separation of the wires, either from 
each other, or from the surfaces, walls, and floors to which they are 
attached. It follows, therefore, that all care should be taken to 
prevent light and power wires carrying currents of large capacity and 
relatively high voltages from coming in contact with signal wires 
since in such event dangerous fires might very readily be caused. 
The same advantages in having wires underground instead of being 
placed on poles apply to signal as well as to light and power circuits, 
but the two classes should never occupy the same underground 
duct, manhole, or handhole even when cables are used, since in the 
mechanical work or repairs on the lines an injury resulting in a cross 
between the systems might cause a dangerous current from the 
higher voltage lines, to outer buildings, over the weakly insulated 
and poorly protected signal wires. The liability of accidental cross¬ 
ing of overhead signaling circuits with electric light and power cir¬ 
cuits may be guarded against to a considerable extent by endeavor¬ 
ing to keep the two classes of circuits on different sides of the same 
street. The Code prescribes that signal wires on pole lines also 
carrying electric light or power wires shall generally be placed on 
the lower cross-arms. This arrangement is not, however, favored 
by many engineers or by all municipal authorities, who prefer that 
signal wires be put on the top arms or on extensions above the 
tops of the poles. The arguments in favor of putting the signal 
wires lowest on the poles are that they are by many judged to be 
more liable to break and fall, especially when loaded with sleet, 
because of their lesser size and strength; also in case of city fire alarm 
circuits which are, of course, very important, the lower position 
enables linemen making repairs to get at the fire-alarm wires without 


142 


UNDERWRITERS’ REQUIREMENTS 


passing up through the light and power wires which may be charged 
with dangerous voltages. 

The arguments in favor of putting signal wires at the top are— 
they are small and, therefore, collect less ice or sleet and so are less 
liable to break; the light and power wires are often better insulated 
and a signal wire breaking is less liable to make a real live contact 
with them; the fall of a heavy power cable may wreck all of the 
signal wires if the latter are below; in the case of city fire-alarm 
circuits the upper location removes them from misuse and injury 
when wiremen are working on the other lines. However, it may be 
said that there is no general agreement either in theory or practice 
on this subject. 

Single wires of signal circuits on the outside of buildings should 
have rubber insulation and where attached to frame buildings should 
be secured to glass or porcelain insulators or knobs. Only copper 
wire should be used for the span from the last pole to the build¬ 
ing and the wires should pass through outside walls through in¬ 
sulating bushings and have drip loops the same as electric light 
wires. Inside of buildings neat arrangement and secure fastening 
of all wires is essential to keep them properly placed and no signal 
wire should come nearer than three inches to any light or power wire, 
unless separated therefrom by some continuous and firmly fixed 
non-conductor creating a permanent separation, this non-conductor 
to be in addition to the regular insulation on the wire as the wires 
would ordinarily be insulated, but the kind of insulation is not 
specified, as the protector described below is relied upon to stop all 
dangerous currents. Porcelain tubing, approved flexible tubing, or 
rigid conduit may be used for encasing wires where required as above. 
Wires where bunched together in a vertical run within any building 
should have a fire-resisting covering sufficient to prevent them from 
carrying fire from floor to floor unless they are run either in non¬ 
combustible tubing or in a fireproof shaft, which shaft should be 
provided with fire stops at each floor. 

Signaling wires and electric light or power wires may be run in 
the same shaft, provided that one of these classes of wires is run in 
non-combustible tubing, otherwise the two classes of wires should be 
separated from each other by at least two inches. In no case should 
signaling wires be run in the same tube with light or power wires. 


UNDERWRITERS’ REQUIREMENTS 


143 


Protecting Devices. In signal installations where the current- 
carrying parts of the apparatus installed are capable of carrying 
indefinitely without overheating a current of ten amperes (as in 
some telegraph or special systems) .the inside wires should be of 
copper at least as large as No. 16 B. & S. gauge and must have the 
same insulation and be supported the same as electric light or power 
wires for 600 volts. At the entrance to the building each wire should 
be protected by a 10-ampere 600-volt fuse. Such signal circuits as 
the above are much less common than those not suited for a 10- 
ampere current. Telephone, district messenger, private watchmen’s 
time recorders, burglar alarms, and fire-alarm circuits, are never 
capable of carrying 10 amperes continuously and for these a special 
“protector” is required located as close as possible to the entrance of 
the building. The purpose of this protector is to prevent any foreign 
current or any lightning discharge from entering the building over 


ft EAT COIL 


~Q> 


rVNS 




FUSE I— 1 —J 

Vwwww 

ARRES 7iW llftlIlr 

FUSE 




|W^W\AA^ 


- \ 


]. l 


TO HOUSE' 
CIRCUIT 


MEAT CO/L 

^GROUND 

Fig. 135. Circuit Showing Proper Placing of Protecting Devices 


the signal wires. For telegraph circuits this protector takes the form 
of a 2,000-volt fuse in each wire. T.he commoner “protector” such 
as is used on telephone lines should have the following parts mounted 
on a porcelain or slate base on which all parts are well insulated: a 
lightning arrester which will operate at 500 volts or more from a 
ground wire not less than No. 18 B. & S. gauge; a fuse in each side of 
the circuit which will blow with small currents (J to 8 amperes) and 
will operate well on the voltages likely to reach the protector in case 
of accident. Where very sensitive instruments are in the circuit, 
such as contain magnet windings which are easily overheated, there 
must be a heat coil on each side of the line. The heat coil is designed 
to warm up and melt out with a current large enough to endanger 
the instruments if continued for a long time, but so small that it 
would not blow the fuses ordinarily found necessary for such instru¬ 
ments. The smaller currents are often called sneak currents. On 
















144 


UNDERWRITERS’ REQUIREMENTS 


those telephone circuits which are supplied with current entirely 
from the central telephone headquarters sneak or heat coils are not 



Fig. 136. Common Form of Telephone Protector 


necessary. The fuses must be so placed as to protect the arrester 
and heat coils, and the protector terminals must be plainly marked 
line, instrument, or ground. 

The relative arrangement of parts is shown in diagram in Fig. 
135 and a common form of protector in Fig. 136. 

TESTING 

Where possible, two tests of the electric wiring equipment should 
be made, one after the wiring itself is entirely completed, and switches, 
cut-out panels, etc., are connected; and another one after the 
fixtures have all been installed. The reason for this is that if a ground 
or short-circuit is discovered before the fixtures are installed, it is 
more easily remedied; and also, because there is no division of 
the responsibility, as there might be if the first test were made only 
after the fixtures were installed. If the test shows no grounds or 
short-circuits before the fixtures are installed, and one does develop 
after they are installed, the trouble, of course, is that the short-circuit 
or ground.is one or more of the fixtures. As a matter of fact, it is a 
wise plan always to make a separate test of each fixture after it is 
delivered at the building and before it is installed. 

While a magneto is largely used for the purpose of testing, it is 
at best a crude and unreliable method. In the first place, it does 
not give an indication, even approximately, of the total insulation 
resistance, but merely indicates whether or not there is a ground or 
short-circuit. In some instances, moreover, a magneto test has 










UNDERWRITERS' REQUIREMENTS 


145 


led to serious errors, for reasons that will be explained. If, as is 
nearly always the case, the magneto is an alternating-current instru¬ 
ment, it may sometimes happen—particularly in long cables, and 
especially where there is a lead sheathing on the cable—that the 
magneto will ring, indicating to the uninitiated that there is a ground 
or short-circuit on the cable. This may be, and usually is, far from 
being the case; and the cause of the ringing of the magneto is not a 
ground or short-circuit, but is due to the capacity of the cable, which 
acts as a condenser under certain conditions, since the magneto pro¬ 
ducing an alternating current repeatedly charges and discharges the 
cable in opposite directions, this changing of the current causing the 
magneto to ring. Of course, this defect in a magneto could be 
remedied by using a commutator and changing it to a direct-current 
machine; but as the method is faulty in itself, it is hardly worth while 
to do this. 

A portable galvanometer with a resistance box and Wheatstone 
bridge, is sometimes employed; but this method is objectioiiable 
because it requires a special instrument which cannot be used for 
many other purposes. Furthermore, it requires more skill and time 
to use than the voltmeter method, which will now be described. 

Voltmeter Method. The advantage of the voltmeter method 
is that it requires merely a direct-current voltmeter, which can be 
used for many other purposes, and which all engineers or contractors 
should possess, together with a box of cells having a potential of 
preferably over 30 volts. The voltmeter should have a scale of not 
over 150 volts, for the reason that if the scale on which the battery 
is used covers too wide a range (say 1,000 volts) the readings might 
be so small as to make the test inaccurate. A good arrangement would 
be to have a voltmeter having two scales—say, one of 60 and one of 
600—which would make the voltmeter available for all practical 
potentials that are likely to be used inside of a building. If desired, 
a voltmeter could be obtained with three connections having three 
scales, the lowest scale of which would be used for testing insulation 
resistances. 

Before starting a test, all of the fuses should be inserted and 
switches turned on, so that the complete test of the entire installation 
can be made. When this has been done, the voltmeter and battery 
should be connected, so as to obtain on the lowest scale of the volt- 


146 


UNDERWRITERS’ REQUIREMENTS 


meter the electromotive force of the entire group of cells. This 
connection is shown in Fig. 137. Immediately after this has been 



Fig. 137. Connections for Voltmeter Test 
Before Circuit is Connected 


done, the insulation resistance 
to be tested is placed in cir¬ 
cuit, whether the insulation to 
be tested is a switchboard, slate 
panel-board, or the entire wiring 
installation; and the connections 
are made as shown in Fig. 138. 
A reading is again taken of the 
voltmeter and the leakage is 
thus obtained, as it is in pro¬ 
portion to the difference be¬ 
tween the first and second voltmeter readings. The explanation 
given below will show how this resistance may be calculated. It is 
evident that the resistance in the first case was merely the resistance 
of the voltmeter and the internal resistance of the battery. As a 
rule, the internal resistance of the battery is so small in comparison 
with the resistance of the voltmeter and the external resistance, that 
it may be entirely neglected, and this will be done in the following 
calculation. In the second case, however, the total resistance in cir¬ 
cuit is the resistance of the voltmeter and the battery,, plus the 
entire insulation resistance on all the wires, etc., connected in circuit. 

To put this in mathematical form, the voltage of the cells may 
be indicated by the letter E; and the reading of the voltmeter when 



Fig. 138. Connections for Voltmeter Test with the Insulation to be Tested in Circuit 


the insulation resistance is connected by the circuit, by the letterE'. 
Let R represent the resistance of the voltmeter and R x represent the 


























UNDERWRITERS’ REQUIREMENTS 


147 


insulation resistance of the installation which we wish to measure. 
It is a fact which the reader undoubtedly knows, that the e. m. f. as 
indicated by the voltmeter in Fig. 138 is inversely proportional to the 
resistance; that is, the greater the resistance, the lower will be the 
reading on the voltmeter, as this reading indicates the leakage or cur¬ 
rent passing through the resistance. Putting this in the shape of a 
formula, we have from the theory of proportion 

or 

Transposing 
and 


E : E' :: R+R x : R 
E' R+E' R X =E R 
E' R x = E R-E' R=R (E—E') 
R (E — E') 


Or, expressed in words, the insulation resistance is equal to the resist¬ 
ance of the voltmeter multiplied by the difference between the first 
reading (or the voltage in the cells) and the second reading (or the 
reading of the voltmeter with the insulation resistance in series with 
the voltmeter), divided by this last reading of the voltmeter. 

Example. Assume a resistance of a voltmeter R of 20,000 ohms, 
and a voltage of the cells E of 30 volts; and suppose that the insula¬ 
tion resistance test of a wiring installation, including switchboard, 
feeders, branch circuits, panel-boards, etc., is to be made, the insula¬ 
tion resistance being represented by the letter R x . By substituting 
in the formula 

R(E-E') 

, E' 

and assuming that the reading of the voltmeter with the insulation 
resistance connected is 5, we have 


Rr 


20,000 X (30-5) 


= 100,000 ohms 


If the test shows an excessive amount of leakage, or a ground or 
short-circuit, the location of the trouble may be determined by the 
process of elimination—that is, by cutting out the various feeders 





148 


UNDERWRITERS’ REQUIREMENTS 


until the ground or leakage disappears, and, when the feeder on which 
the trouble exists has been located, by following the same process 
with the branch circuits. 

Of course, the larger the installation and the longer and more 
numerous the circuits, the greater the leakage will be; and the lower 
will be the insulation resistance, as there is a greater surface exposed 
for leakage. The rules of the Code give a sliding scale for the require¬ 
ments as to insulation resistance, depending upon the amount of 
current carried by the various feeders, branch circuits, etc. The rule 
of the Code covering this point, is as follows: 

The wiring in any building must test free from grounds; i. e., the com¬ 
plete installation must have an insulation between conductors and between 
all conductors and the ground (not including attachments, sockets, recepta¬ 
cles, etc.) not less than that given below: 


Up to 

5 amperes. 

. 4,000,000 ohms 

Up to 

10 amperes. 

. 2,000,000 ohms 

Up to 

25 amperes. 

. 800,000 ohms 

Up to 

50 amperes. 


Up to 

100 amperes. 

. 200,000 ohms 

Up to 

200 amperes. 


Up to 

400 amperes. 

. 50,000 ohms 

Up to 

800 amperes. 

. 25,000 ohms 

Up to 1,600 amperes. 



The test must be made with all cut-outs and safety devices in place. If 
the lamp sockets, receptacles, electroliers, etc., are also connected, only one- 
half of the resistances specified in the table will be required. 

DEVICES AND MATERIALS 

No care in installing electrical equipments will entirely com¬ 
pensate for the use of inferior or defective devices or materials. 
The National Board of Fire Underwriters has for many years main¬ 
tained a system of tests and examinations of electrical appliances, 
and issues twice a year a “List of Electrical Fittings” which con¬ 
tains in a classified form under the names of their manufacturers all 
of the standard and special fittings and materials which have been 
approved. These tests and examinations are made and the approvals 
are issued by Underwriters’ Laboratories, Inc., a thoroughly equipped 
institution maintained in Chicago by the fire insurance interests, for 
the express purpose of examining and testing all kinds of devices and 
materials, electrical and otherwise, which have any bearing what- 











UNDERWRITERS’ REQUIREMENTS 


149 


soever on the fire hazard. This “List of Fittings” issued semi¬ 
annually, is universally recognized as the only complete and reliable 
guide to properly safeguarded electrical appliances and it should be 
consulted and followed in the choice and purchase of supplies. The 
Underwriters’ Laboratories also maintain an elaborate system 
whereby manufacturers may obtain and place on their wares special 
labels issued by the Laboratories, which thus become a guarantee on 
the goods themselves that the articles bearing such labels are in con¬ 
formity with underwriters’ rules and have been examined and tested 
by special underwriters’ inspectors at the factories where they are 
made. This system of label service and inspection is not yet extended 
to all classes of approved electrical devices, but for those classes of 
appliances which are now included, the label may be regarded as 
affording to prospective purchasers and users reliable evidence not 
only of the general approval of the design but also that the particular 
sample bearing the label is made in accordance with requirements 
and is suitable for use. 

| The constructional details of electrical fittings and materials 
together with the chief tests to which they are subjected, prior to 
approval, are contained in what is known as “Class D” of the Code. 
This pamphlet should be consulted for full details or inquiry should 
be addressed to Underwriters’ Laboratories, 207 East Ohio Street, 
Chicago, Illinois, for information not given in the Code or in the 
semi-annual “List of Fittings.” 

In the following pages is given a brief discussion of the chief 
characteristics and requirements of some of the more common 
materials and of the more important classes of devices used in elec¬ 
trical construction work. 

■Vv 

RUBBER=COVERED WIRE 

General Specifications. A considerable variety of grades of 
rubber-covered wire is manufactured, some makers offering several 
grades and others only one or two at most. The chief distinction 
lies in the quality and quantity of real new, pure, fine rubber gum 
used in the compound. It is not possible to determine or grade the 
excellence of a rubber compound by any direct or readily applied 
tests, but somewhat elaborate tests, physical, chemical, and elec- 


150 UNDERWRITERS' REQUIREMENTS 

TABLE IV 

Thickness of Rubber Insulation 


B. & S. Gauge 

Thickness 

18 to 16 

1-32 inch 

15 to 8 

3-64 inch 

7 to 2 

1-16 inch 

1 to 0000 

5-64 inch 

Circular Mils 

• 

250,000 to 500,000 

3-32 inch 

500,000 to 1,000,000 

7-64 inch 

Over 1,000,000 

1-8 inch 


trical, are necessary to arrive at any correct estimate. The following 
properties of good wire may, however, be noted. The rubber should 
be neither hard and dry, nor soft and spongy. When examined 
minutely it should appear of a close uniform texture free from small 
bits of unmixed matter or pinholes. It should adhere closely to the 
tinned copper. The thickijess of rubber wall should correspond to 
the data given in Table IV. 

Measurements of insulating wall are to be made at the thinnest 
portion of the dielectric and it should be very carefully noted whether 
the copper is exactly centered in the rubber covering so that the full 
prescribed insulation is maintained on all sides. 

The rubber insulation should exhibit a fair degree of elasticity 
when pieces cut from the wire are stretched and released. If the 
rubber breaks with a very slight pull and shows no ability to stretch 
and recover its first length it is probable that a very poor grade of 
gum has been used, or that the manufacturing process is defective, 
or both. 

After the braid has been carefully removed it should be possible 
to wind the smaller sizes of wire about a cylinder of the same diameter 
as the rubber-covered wire without the rubber showing any breaks 
or cracks either at once or after several days. 

The foregoing should be considered as only rough tests and not 
susceptible of exact application except under conditions which can 
be maintained in a regular testing laboratory. No directions can be 
given which will permit any but an expert chemist to make chemical 
examinations of rubber compounds. 










UNDERWRITERS’ REQUIREMENTS 


151 


The braids should be of close weave and should be very thor¬ 
oughly saturated with the compound. All wires should have through 
their entire length a marker indicating by whom they were made 
and each coil should bear a tag giving, beside the maker’s name, the 
maximum voltage for which it is designed, the words “National 
Electrical Code Standard,” and the month and year when it was 
manufactured. Every coil of approved wire is separately tested by 
the maker at the factory by at least two electrical tests, one designed 
to show that the insulation is free from mechanical defects and the 
other to show at least a minimum insulation value. 

Special Insulation. Most makers are prepared to furnish at a 
special price a grade of wire commonly known as “thirty per cent 
Para.” This wire is understood to have an insulation containing at 
least 30 per cent of “fine, pure, up-river Para” gum which is much 
more than common commercial rubber-covered wire contains. This 
30 per cent wire is also made according to certain rather exacting 
specifications designed to insure a high grade of insulation with good 
lasting'properties. Wire of this description is often specified where 
an extra good quality is desired for first class work. 

A good compound should contain a large percentage of pure, 
fine, new rubber of excellent quality. Para rubber is universally 
admitted to be the best for imparting life, strength, and durability 
to the insulation. The use of reclaimed rubber or any of the so-called 
rubber substitutes reduces the excellence of the compound approxi¬ 
mately in the proportion in which it is used. The other ingredients 
of a good compound are solid, waxy, hydrocarbons, suitable mineral 
matter and sulphur. The sulphur plays an important part in the vul¬ 
canizing of the compound, the process whereby the rubber is trans¬ 
formed from its original and almost crude state into the substance 
familiar to us as manufactured rubber in any one of its numerous forms. 

Fixture Wire. Fixtures may be wired with flexible cord or 
standard rubber-covered wire, and for other wires for use in fixtures, 
the following rules apply: The wire may be either solid or stranded 
and not less than No. 18 B. & S. gauge. Solid conductors must be 
tinned and stranded conductors must be of strands not less than 
No. 30 B. & S. gauge and must have a cotton wind between copper 
and rubber. The No. 18 wire may have a rubber insulation Fi-inch 
thick, but No. 16 and also flexible cord used in fixtures must have 


152 


UNDERWRITERS’ REQUIREMENTS 


at least 32 -inch rubber. All sizes must be covered with a good 
braid. The concession of so thin a rubber wall on No. 18 wire has 
been made because a very small wire must be used to pass through 
the arms and other parts of many fixtures. 

In wiring certain designs of show-case fixtures, ceiling bull’s- 
eyes and similar appliances, in which the wiring is exposed to tem¬ 
peratures in excess of 120° F. (49° C.), from the heat of the lamps, 
slow-burning wire may be used. 

Insulation for Conduit and Armored Circuits. For all conduit 

work and in all armored cable the wire is regular standard rubber- 
covered with an extra braid. For twin or duplex wires, this outer 
braid, which should be at least it- inch thick, is made as a covering 
over the two regular rubber-covered and braided conductors. These 
twin wires are generally used in conduit work but where single con¬ 
ductors are used they must also have double braid. The purpose of 
this extra outer braid is primarily to withstand the abrasion and 
strain, resulting from hauling the conductors through the conduits 
from outlet to outlet, and the braids on the individual conductors 
are to hold the rubber insulation in place and prevent jamming and 
flattening which might reduce the thickness of rubber between the 
two wires and thus weaken the insulation at many points. 

RIGID CONDUIT AND CONDUIT FITTINGS 

Unlined Steel Conduit. The following description applies 
only to standard unlined steel conduit. This is made of mild steel 
with a butt weld joint lengthwise of the pipe. Sizes run from normal 
J-inch to 4-inch pipe. The raw pipe is thoroughly cleaned inside 
and outside and then given a protective coating either of an enamel 
baked on or of zinc applied either by electroplating or by a special proc¬ 
ess known as sherardizing. With either form of zincing the interior 
is given a coat of enamel also. The finished pipe should be smoothly 
coated and able to stand bending without injury to the enamel or the 
zinc. The conduit should be of sufficiently true circular section to 
admit of cutting true, clean threads. The enamel applied to con¬ 
duit is not considered as an insulation but either enamel or zinc is 
required to protect the steel from rusting away and also to give a 
smooth surface for the conductors to be drawn over in inserting the 
wires. Ordinary commercial pipe should never be used as electric 
conduit since it is not free from rough edges, is not maintained at 


UNDERWRITERS’ REQUIREMENTS 


153 


TABLE V 

Minimum Weights of Conduit for Required Wall Thickness 


Size 

Inches 

Pounds per 

100 Feet 

Size 

Inches 

Pounds per 

100 Feet 

1 

2 

75 

H 

250 

3 

4 

104 

2 

350 

1 

152 

3 

710 

H 

209 




uniform size or wall thickness, is not protected against rust and in 
general is not made with the care and rigid inspection which have 
been found by experience to be necessary for electric conduits. 
Table V gives the minimum weights per 100 feet of finished conduit 
which are required to give the specified thickness of wall. 

Conduit Fittings. A very great variety of boxes and small fittings 
for use with conduit is available. All boxes including flush switch 
boxes should be either of cast iron with walls at least J-inch thick or 
of sheet steel at least .078 inch thick. They must be well enameled 
or galvanized to protect them from rusting and must have no open¬ 
ings not closed by the entering pipes, by a metal cover of the same 
thickness of the box or by the switch, receptacle, or canopy of the 
device attached to them. Under no circumstances is it allowable to 
place any such box so that it will not be accessible. There should be 
no rough edges or corners which are liable to injure the coverings of 
wires as they are drawn in. Fig. 139 shows the form of a common 
type of box. Boxes for use with combina¬ 
tion gas and electric fixtures must be pro¬ 
vided with an arrangement for making a 
tight electrical connection between the 
box and the gas pipe at each outlet so 
that there may be no arcing between box 
and pipe in case any failure of wire insu¬ 
lation causes a current to flow over the 
box. Otherwise such an arc may burn a 
hole in the gas pipe and ignite the gas. 

All threaded parts of boxes and all threads 
on locknuts and metal bushings must be clean cut and well fitted in 
order to insure that permanent and reliable electrical continuity of the 
conduit system which is one of the chief requirements for conduit work. 



















154 


UNDERWRITERS’ REQUIREMENTS 


Set screw connections have been found unsatisfactory as they 
loosen with the vibration of buildings and with changes of tempera¬ 
ture, and only regular screwed thread joints or substantial clamps 
should be used at all conduit and armored-cable connections. 



Fig. 140. Common Form of Floor Outlet Box 


Where a floor outlet in a conduit system is desired, a special 
type of box should be used known as a floor outlet box. Such boxes 
(see Fig. 140) provide ample room for making splices in wires, for 
mounting receptacles or other fittings and especially provide a sub¬ 
stantial, watertight top or cover which can be set flush with the floor 
surface. The practice sometimes followed of setting flush wall 
receptacles in floors is to be strongly condemned since such fittings 
are not strong enough for such service and are not watertight, thus 
permitting water to enter the conduit box and system. 

FUSES OR CUT=OUTS 

Classification. Three forms of fuses are at present employed 
in this country for general wiring work, open-link fuses, cartridge fuses 
apd plug fuses, the last two being further described as enclosed fuses 
to distinguish them from the open links. The bases to which or in 
which the fuses are secured are called cut-outs. 

Link fuses are extensively used on large switchboards and their 
use on such boards is open to less objection than for general wiring 
since such boards are usually under expert supervision and located 
in well-protected or fireproof rooms. With link fuses there is always 
the possibility of a larger fuse being put into the cut-out than it was 
designed for, which is not true of enclosed fuse cut-outs classified as 




UNDERWRITERS’ REQUIREMENTS 


155 


TABLB VI 

Open=Link Fuse Spacing 



Minimum Separation of 
Nearest Metal Parts 
of Opposite Polarity 
Inches 

Minimum 

Break 

Distance 

Inches 

125 Volts or less 



10 amperes or less 

3 

4 

1 

4 

11-100 amperes 

1 

3 

4 

101-300 amperes 

1 

1 

301-1000 amperes 

U 

1 ? 

126 to 250 Volts 



10 amperes or less 

u 

n 

11-100 amperes 

it 

u 

101-300 amperes 

2 

ii 

301-1000 amperes 

21 

2 


required below. Again, the voltage in most plants can, under some 
conditions, rise considerably above the normal. The need of some 
margin, as a factor of safety to prevent the cut-outs from being 
ruined in ordinary service, is therefore evident. When tablet-boards 
or single fuse-blocks with such open-link fuses on them are used in 
general wiring, they must be enclosed in cabinet boxes. This is 
necessary, because a severe flash may occur when such fuses melt, 
so that they would be dangerous if exposed in the neighborhood of 
any combustible material. Link fuses should never be mounted on 
porcelain cut-outs because a severe short-circuit is liable to break 
this rather fragile material and the molten metal is apt to fuse into 
the porcelain, partly reducing its insulating properties. 

There is no filler surrounding the fusible metal of open links 
and, therefore, the ability of the fuse to open the circuit depends 
on having enough of the metal burned away, when the fuse blows, to 
break the arc. For this reason the terminals for link fuses, as far 
as practicable, should be made of compact form instead of being 
rolled out in thin strips; and sharp edges or thin projecting pieces, 
as on wing thumb nuts and the like, should be avoided. Thin metal, 
sharp edges, and projecting pieces are much more likely to cause 
an arc to start than a more solid mass of metal. It is a good plan 
to round all corners of the terminals and to chamfer the edges. Plain 
fuse wire or fuse strip should never be used for links but only fuses 
made up with solid metal terminals as shown in Fig. 17. 






156 


UNDERWRITERS’ REQUIREMENTS 


In general work open-link fuses should not be used on circuits 
of voltages above 250 volts and where large currents are involved 
the use of approved circuit breakers is very much to be preferred to 
extra large link fuses. The spacings for open-link fuses are given in 
Table VI. 

Plug Fuses. The form of fuse which is used in larger num¬ 
bers than those of any other type, is shown in Fig. 141. Fig. 142 
shows one of the many forms of cut-out bases for plug fuses and 
Fig. 57 shows similar blocks arranged in an asbestos-lined cabinet 
to make a tablet or panel-board for distributing current to branch¬ 
lighting circuits. The cabinet should have a tightly fitted asbestos- 
lined or metal-lined door. Plug fuses are approved for use only on 
circuits of not over 125 volts, including 3-wire circuits with grounded 
neutral, and not over 250 volts between outside wires. Fig. 143 



Fig. 141. Standard Fuse Plug Fig. 142. Cut-Out Base for Fuse Plugs 


shows the effect if the fuses are blown on a 220-volt circuit. These 
plugs are limited to ratings of 30 amperes and less because their 
form and strength is not such as to make them safe for use with larger 
currents. They are, therefore, chiefly adapted for small lighting and 
motor branch circuits. They are decidedly safer than open-link 
fuses of equal capacity and are cheaper than enclosed cartridge fuses. 

There are a number of patterns of unapproved plug fuses on 
the market which should be avoided as they usually lack some of the 
essential properties of safe fuses, although they may appear from 
casual inspection to be almost identical with them. It is unfor¬ 
tunately true that plug fuses of the present form can be “doctored” 
in several ways so as to carry larger currents than they should. Tin- 
foil, solder, and bits of copper wire are often found put around or into 
plug fuses so as to completely destroy their usefulness as protective 
devices. Inspectors and property owners should be on the lookout 
for this highly dangerous practice and also observe carefully whether 
plugs of too large current capacity have been substituted for those of 





UNDERWRITERS’ REQUIREMENTS 


157 


ratings which afford real protection to the wiring of the circuits of 
which they are a part. 



Fig. 143. Effect of Plug Fuses Blown on 220-Volt Circuit 

Cartridge Fuses. A cartridge fuse consists of a cylindrical tube 
of fiber or strong paper to which are fitted metal caps by means of 
which connection is made to the cut-out terminals. Wfithin the tube 
is the fusible metal wire or strip extending between and firmly secured 
to the inside of the caps. The tube is filled with a powdered or 



0-30 AMPERES 250 VOLTS 







5 sag o -gQBSCTPa'op < 

Co oi $ 

ilo o il is 






I0l~200AMPERES 2S0 VOLTS 

Fig. 144. Sections through Cartridge Fuses Showing Construction 


granulated material packed closely about the fusible strip. The 
purpose of this filler is to conduct the heat from the strip to the outer 

































158 


UNDERWRITERS’ REQUIREMENTS 


casing, to smother the arc when the fuse blows, to make it possible 
to adjust more exactly the carrying capacity of the fusible strip, and 
to absorb some of the gases evolved from the molten metal. The chief 
ingredients of fillers now used are magnesia and plaster of Paris. 
Fig. 144 shows in section the internal construction of two typical 
cartridge fuses of 30 and 200 amperes capacity. The illustration 
is full size. The small wire extending from one terminal to a point 
on the tube and thence to the other terminal is designed to burn 
off at the tube when the fuse blows and fuse a bit of powder under 
a slip of thin paper on the outside of the casing, thus indicating that 
the fuse has operated. As will be seen from the description a cart¬ 
ridge fuse will confine the arc, flame, or molten metal within the filler 



STYLE OF TERMINAL FOR CARTR/D6F STYLE OF TERMINAL FOR CARTRJD6E 

FUSES 0-60 AMPERES. FUSES 61-600 AMPERES 


FORM I. FERRULE CONTACT FORM 2. KN/FE-BLADE CONTACT 


VOLTA 6 E 

RATED 

CAPACITY 

AMPERES 

A 

B 

C 

D 

E 

F 

G 

RATED 

CAPACITY 

AMPERES 

LEN6TH 

OVER 

TERMINALS 

INCHES 

DISTANCE 

BETWEEN 

CONTACT 

CLIPS 

INCHES 

WIDTH 

OF 

CONTACT 

CLIPS 

INCHES 

DIAMETER OF 
FERRULESOR 
THICKNESS 
OF TERMINAL 
BLADES 

INCHES 

MIN. LENGTH 
OF FERRULES 
OR OF TER¬ 
MINAL BLADES 
OUTSIDE OF 
TUBE 
INCHES 

DIAMETER 
OF TUBE 

INCHES 

WIDTH OF 
TERMINAL 
BLADES 

INCHES 


0-30 


2 

. 1 

'/? 

Via . 

Vz 

Vz 



0-30 

0-250 

31-60 

1 

3 

IV* 

Va 

'Via 

Va 

V*. 


1 

31-60 


61-100 

CM 

5 7 /a 

4 

Va 

'/a 

1 

I 

% 

CM 

61-100 


101-200 


7 'fa 

4'lz 

I'M 

Via 

IVa 

U/z 

1 Va 


101-200 


201-400 

§ 

8Va 

5 

IV* 

'/* 

IVa 

2 

I s '/a 

§ 

201-400 


401-600 


10 Va 

6 

2 Va 

V* 

2 V* 

2Vz 

2 

£ 

401-600 


0-30 

$ 

5 

4 

Vz 

'Via 

'/z 

V* 



0-30 

251~bOU 

31-60 


5'lz 

4'U 

■ Va 

/ 'ha 

Va 

I 



31-60 


61-100 

rvj 

7 Va 

6 

Va 

Va 

1 

!'/* 

. % 

CM 

61-100 


101-200 


9 s / a 

7 

I'M 

Via 

IVa 

IV* 

IVa 


101-200 


201-400 

1 

II 5 la 

8 

PM 

V* 

IVa 

2'/z 

/•% 

5 

201-400 


Fig. 145. Dimensions and Classifications of Cartridge Fuses 


and tube when its fusible element is melted by an overload or by a 
short-circuit and is, therefore, a safer device than an open-link fuse. 

In 1905 a standardization of cartridge fuses and their cut-out 
bases was agreed upon in order to bring them all to uniform dimen¬ 
sions, to arrange the different ratings under a classification which 
would make it impossible to put a large fuse into a cut-out base of 
a smaller class, and to make it possible to use any approved fuse in 
any approved base whether fuse and base were of the same or different 
make. Under this classification two types of terminals were stand- 





































































UNDERWRITERS’ REQUIREMENTS 


159 


ardized known as ferrule and knife-blade contacts, and the current 
ratings were grouped into two sizes for the ferrule and four sizes 
for the knife-blade contacts. The dimensions and classification 
are fully shown on the diagrams and in the table, Fig. 145. 

Knife Switches. Knife switches must always be mounted on 
separate bases of slate, marble, or porcelain, or on slate or marble 
switchboards or panels. The parts carrying the contact clips and 
the blade hinges must be secured to the base by two screws, a screw 
and a dowel-pin, or otherwise, so that the parts will always be in 
correct alignment. If the contact jaws or hinge clips get turned so 
as to be out of line, it may be impossible to close the switch, espe¬ 
cially at the first attempt, and severe arcing may result from the 
efforts to do so. Even if the blade enters the jaws, the contact may 
be imperfect, causing undesirable heating. The chief points to note 
in judging a knife switch are the following: Excellence of fit of 
blades both at the hinge and in the contact clips; stiffness and size 
of all metal parts to secure good contact surfaces and ample carrying 
capacity. No part should become heated over 50° F. when the 
switch is carrying its full rated current, and at all sliding contacts 
there should be at least 1 square inch of surface contact for every 
75 amperes of current. The cross bars should be very securely fas¬ 
tened to the blades and the workmanship throughout should be 
excellent. If each blade is secured to the cross bar by only one screw, 
without dowel-pins or a square shoulder fitting closely in a recess in 
the bar, a slight loosening of the screws will allow one blade to close 
and open the circuit before the other, resulting in arcing and ultimate 
injury to the switch. Such construction is also liable to result in a 
weak switch. Too little attention is frequently given the question 
of mechanical strength, with the result that after a comparatively 
short time of service the switches rattle to pieces or break unless 
very carefully handled, and even then repairs are often necessary 
to keep them in working order. A cheap switch is seldom a rugged, 
durable device. All switches should be marked with the name of 
their maker and the rating in both volts and amperes. 

The spacings of switches must be at least as great as those given 
in the Code, a copy of which is given in the following table. This 
table specifies the limits necessary for both direct-current and 
alternating-current systems. 


160 


UNDERWRITERS’ REQUIREMENTS 


TABLE VII 

Approved Spacing for Knife Switches 


• 

Minimum Separation 
of Nearest Metal 
Parts of Opp. Pol. 
Inches 

Minimum 

Break 

Distance 

Inches 

Not over 125 Volts D. C. and A. C. 



for Switchboards and Panel Boards* 

♦ 


10 amperes 

3 

4 

1 

2 

30 amperes 

1 

3 

4 

60 amperes 

if 

1 

Not over 125 Volts D. C. and A. C. 
for Individual Switches! 



30 amperes 

if 

1 

60 and 100 amperes 

li 

U 

200 and 300 amperes 

21 

2 

400 and 600 amperes 

2f 


800 and 1,000 amperes 

3 

2f 

250 Volts only D. C. and A. C. for 
All Switches 



30 amperes 

If 

H 

Not over 250 Volts D. C. nor over 



500 Volts A. C. for All Switches! 



30, 60 and 100 amperes 

2f 

2 

200 and 300 amperes 

2* 

2f 

400 and 600 amperes 

2f 

2f 

800 and 1,000 amperes 

3 

2f 

Not over 600 Volts D. C. and A. C. 



for All Switches** 

• 


30 and 60 amperes 

4 


100 amperes 

4 1 



*The 10-ampere switch must have ample metal for stiffness, and to prevent rise in 
temperature of any part of more than 50 degrees Fahrenheit when carrying 30 amperes, the 
contacts being arranged so that a thoroughly good bearing at every point is obtained with 
contact surface advised for pure copper blades of about 0.4 square inch. 

fThe 300-ampere switch must not be equipped with cut-out terminals. 

JThe above switches must be stamped “250 V. D. C., 500 V. A. C.” The 30-ampere 
switch must have ample metal for stiffness, and to prevent rise in temperature of any part 
of more than 50 degrees Fahrenheit when carrying 60 amperes, the contacts being arranged so 
that a thoroughly good bearing at every point is obtained with contact surfaces advised for 
pure copper blades of about 0.8 square inch. The 300-ampere switch must not be equipped 
with cut-out terminals. Cut-out terminals on switches for over 250 volts must be designed 
and spaced for 600 volt fuses, and in such cases the switches must be stamped “500 V. A. C.” 

**The 30-ampere switch must have ample metal for stiffness, and to prevent rise in 
temperature of any part of more than 50 degrees Fahrenheit when carrying 60 amperes, the 
contacts being arranged so that a thoroughly good bearing at every point is obtained with 
contact surfaces advised for pure copper blades of about 0.8 square inch. 

Auxiliary breaks or the equivalent are recommended for D. C. switches designed for over 
250 volis and must be provided on D. C. switches designed for use in breaking currents 
greater than 100 amperes at a voltage of over 250. 

Note. For three-wire direct-current and three-wire single-phase systems 
the separations and break distances for plain three-pole knife switches must 
not be less than those required in the above table for switches designed for the 
voltage between the neutral and outside wires. 





















UNDERWRITERS’ REQUIREMENTS 


161 


Snap Switches. Under this term are included the common 
round base surface switches, the rotary and push-button switches, 
which are set in boxes in side walls flush with the surface, pendant 
switches, and all such as are operated by the motion of doors, by a 
cord and all switches mounted on fixtures. The distinguishing 
feature of them all consists in the fact that the motion of the parts 
which open and close the circuit is produced by a spring contained 
in the mechanism. As the handle or button is turned or pushed 
this spring is wound up and at the proper tension is released, thus 
throwing the switch blades into or out of the contacts. Thus the 
quickness w T ith which the circuit is opened or closed is not directly 
determined by the motion of the operator’s hand but by the spring, 
and if the switch is of proper design and in good condition, the 
action is prompt and reliable even though the person using the 
switch is not careful to do just the right thing. Such snap switches, 
therefore, differ from knife-blade switches in that their proper use 
does not depend upon the user and they are correspondingly better 
suited for general purposes for unskilled persons. 

The bases should be of non-combustible material, usually 
porcelain, and all covers should be lined with a non-conducting 
material such as fiber unless they are of porcelain. Without this 
lining there is danger of the cover forming a short-circuit in the 
switch, especially if the cover is removed or replaced while the 
switch is “alive.” The side lining should extend beyond the lower 
edge of the cover. 

The binding posts should be of a type in which the end of the 
connected wire is held under a screw head or equivalent device and 
not by a set-screw the end of which drives against the side of the 
wire, as a set-screw is likely to become loosened and is almost sure 
to cut into the wire. Indicating switches are much preferred for all 
work, as by showing at once whether the current is “on” or “off” 
they tend to save mistakes and possible accidents. The fact that 
lights do not burn or that a motor does not run is not necessarily a 
sure sign that the current is off, but the indicating switch makes it 
possible to tell at a glance whether the circuit is open or closed. 

Fig. 146 shows a variety of approved snap switches of common 
type. Snap switches to be approved are required to operate success¬ 
fully at 50 per cent excess current above that for which they are 


162 


UNDERWRITERS’ REQUIREMENTS 


rated at the voltages for which they are designed. This is to provide 
a margin of safety but should not be made an excuse for using them 
for larger currents than those marked on them. They are made in 
a great variety of sizes and patterns, single- and double-pole, three- 
and four-way, and in special designs for turning on the lamps of a 
chandelier one after the other or for controlling small motors or heat¬ 
ing devices. The standardized ratings include 125, 250, and 600 
volts for currents of 3, 5, and 10 amperes in the more commonly 
used patterns. Certain large-sized double-pole surface snap switches 
are rated at 20 or 30 amperes while a few of the more special and less 
substantial forms are limited to 1 ampere only. In judging snap 



Fig. 146. Group of Approved Snap Switches 


switches of all kinds samples are tested by Underwriters’ Labora 
tories in the following way: they are connected to control groups of 
lamps taking full rated current of the switch at full rated voltage 
and are then put on a special machine which operates them slowly 
and continuously for 6,000 cycles, that is 6,000 full “on and off” 
operations. It is required that the samples stand this endurance 
test without failing either mechanically or electrically. This test is 
re-applied to new and recent samples from time to time, and similar 










UNDERWRITERS’ REQUIREMENTS 


163 


tests are constantly in progress at the factories where the switches 
are made, so that any important defect in material or construction 
is soon detected and a fairly uniform grade of snap switches is sure 
to be produced for the user. Such persistent tests have done much 
to improve the quality of these and other electrical products. 

CIRCUIT BREAKERS 

Circuit breakers are automatic switches so designed that an 
excess current will cause the switch to open. They thus share the 
properties of both switches and protective devices such as fuses, and 
in their choice and installation both functions must be considered. 
Breakers are made for currents of all capacities and for circuits of 
every voltage both d. c. and a. c. Those most commonly used on 
lighting and power switchboards and in general commercial work 
are for voltages of 600 volts or less (occasionally 2,000 volts). For 
higher voltages the breakers are of massive form and are often set 
in cells or compartments of brick, slate, or concrete. Such breakers 
are employed only where expert supervision is always available and 
their form and operation is, therefore, not prescribed by under¬ 
writers’ rules but rather by the engineering necessities of the system 
of which they are a part. 

The ordinary commercial breaker as used on low-voltage cir¬ 
cuits may be one, two, or three poles and these may be independent 
of each other or may be interlocked so that an overload on any one 
wire will cause all the lines to be opened. The latter is preferable. 
Breakers are usually made with an adjustment regulating the point 
at which they will open. Thus a 100-ampere breaker may be set 
to open at any current with a certain range above and below 100 
amperes. 

In installing breakers the same care should be taken as with 
fuses of like capacity. They should never be placed near any in¬ 
flammable material, as their operation under severe overloads 
results in a severe though brief arc and often in the spattering of bits 
of molten metal quite capable of igniting waste, shavings, etc. The 
use of circuit breakers instead of fuses is to be recommended for 
very large currents and for all circuits such as many motor circuits 
where operating conditions are liable to produce frequent overloads. 


164 


UNDERWRITERS’ REQUIREMENTS 


Classification. Circuit breakers may be divided into two chief 
classes, carbon or air-break patterns and oil-immersed patterns. The 



Fig. 147. Modern Oil Circuit Breaker 


former may be used on either d. c. or a. c. circuits. They have 
copper blades and fixed contacts with carbon secondary contacts 
arranged to open just after the heavy copper contacts. Thus the 
current is carried by copper parts of ample size when the breaker is 
closed, and the arcing on opening is largely confined to the carbon 
contacts which are better for this purpose than metal. Such breakers 
may become dangerous either from overheating of the coils, from 





1 


—y—bF 

— □ — fp 


— , 

s — 

=S=3» 

C=Q=' 

» ( 


-fp-tp 



l - 



Fig. 148. Typical Panel-Board Bus-Bar Arrangements 


arcing upon opening heavy currents, or from failure to act in emer¬ 
gency as they are intended to do. However, the better types of 
modern breaker are very well made and form reliable protective 
devices. 
















































UNDERWRITERS’ REQUIREMENTS 


165 


Oil-immersed breakers are coming more and more into use. 
They are made with their contacts immersed in a heavy oil contained 
in a can or case of suitable*form. When the contacts open, the oil 
aids very greatly in quenching the arc, and such breakers can, 
therefore, be made quite compact since a long break distance is not 
so essential. These breakers are made both for switchboards and 
wall mounting. They are not so well suited to direct-current circuits 
since the action of the d. c. arc carbonizes the oil too rapidly. Fig. 
147 shows a modern oil breaker. They present a hazard due to the 
oil which may become overheated or even ignited. This is unlikely 
to occur with a well designed oil switch or breaker, but the usual 
precautions should be taken to keep the switch and its immediate 
neighborhood clean and free from accumulations of rubbish or any 
inflammable material which may become oil soaked. 

MISCELLANEOUS DEVICES 

Panel Boards and Cabinets. Panel boards are distributing 
boards, or switchboards from which the branch circuits are led off 
from the mains. They must have slate bases on which are mounted 
the necessary bus bars, switches and fuses. A very great variety 
of panels is made, the arrangement of parts in a few patterns 
being shown in Fig. 148, while a complete panel set in a steel 
cabinet is shown in Fig. 149. Fig. 150 shows the wiring channel 
often provided and a cabinet with wood door and trim. The 
panel base and the two partitions shown in the section drawing 
are of slate and all other interior surfaces of the cabinet including 
the door should be lined with sheet steel. 

Wood cabinets should not be used on conduit, armored 
cables, or metal molding systems of wiring as they do not form a 
metallic connection between parts of the system and any attempt 
to overcome this by bonding around the box by wire is not liable 
to result in a good job. Metal cabinets are preferable in all cases 
except possibly in very damp locations where they are liable to 
rust rapidly. All cabinets whether for panels or for individual 
switches or cut-outs should be thoroughly dust tight and fitted with 
tightly-closing doors. No metal thinner than No. 16 U. S. Metal 
Gauge should be used and heavier metal is necessary for all but the 
smaller sizes of box to secure the requisite stiffness and durability. 


166 


UNDERWRITERS’ REQUIREMENTS 


Sockets and Receptacles. The almost endless variety of these 
fittings may be classified in various ways, as for dry or for wet places; 
key and keyless types; brass shell or porcelain types; conduit boxes; 
molding signs; miniature and candelabra sockets and receptacles, 
etc. All standard sockets and receptacles are now made with what 



Fig. 149. Complete Panel Board in Steel Cabinet 


are called Edison screw-shells into which the base of the incandescent 
lamp is screwed, the shell being connected in the socket, to one of the 
lead wires and the center contact to the other lead wire. In all types 
the design of the socket or receptacle should be such that when a 
lamp is inserted there will be no current-carrying part of the lamp 
base exposed. This calls for a minimum depth of the socket of H 
inch and sockets and receptacles which do not have such depth should 
not be used. Sockets and receptacles were formerly rated in candle- 













UNDERWRITERS’ REQUIREMENTS 


167 


power of the lamps designed for them or in amperes and volts but the 
introduction of the newer high-efficiency lamps such as the tungsten 
and tantalum has rendered this method of rating inapplicable. All 
key or pull sockets and receptacles of standard types are now rated 
250 watts, 250 volts, with a provision that this shall not be inter¬ 
preted to permit the use at any voltage of current above amperes. 

Keyless sockets and receptacles of standard types are rated 
660 watts, 250 volts, but not over 6 amperes at any voltage. 

Miniature and candelabra sockets are rated 75 watts, 125 volts. 



Fig. 150. Panel Board and Wood Cabinet Showing Method of Construction 


Weatherproof sockets having no exposed current-carrying parts 
may be rated 660 watts, 600 volts, and thus may be used in series' 
on 600-volt circuits. 

The most common abuse of sockets is to employ them as out¬ 
lets for currents far in excess of what they or the wiring immediately 
connected to them should carry and the above limits should be 
rigidly adhered to. The assigned ratings do not imply that the full 






















































































168 


UNDERWRITERS’ REQUIREMENTS 


power can be taken from each and every socket on a circuit at once. 
Thus twelve carbon 16 c. p. lamps in sockets may take 6 amperes 
on a 110-volt circuit and be fused with 6-ampere fuses. At 250 watts 
per socket, twelve sockets will take 3,000 watts which, at 110 volts, 
is over 27 amperes, an amount of power forbidden by the rules 
limiting lighting branch circuits to 660 watts and a current mani¬ 
festly too large for the No. 14 wire which would usually be used on 
such a circuit. The ratings 250 watts, 250 volts, for key sockets, and 
660 watts, 250 volts, for keyless sockets, are intended to express the 
maximum safe carrying capacity of each socket or receptacle alone 
and do not warrant employing them in the manner indicated above, 
which would seriously overload ordinary circuits. 



Fig. 151. Common Types of Fused and Unfused Rosettes 


Rosettes. These devices are usually of porcelain and provide 
a means of connecting flexible cords to the main or branch circuits. 
Types are made either with or without small link fuses in the base 
but the unfused type is much to be preferred and should be used 
exclusively in general work since the use of link fuses in porcelain 
fittings is undesirable because of the possible results following a short- 
circuit blowing the fuses violently. It is much better to place all 
fuses at distribution centers, such as panel boards, and by keeping 
the fuses of proper capacity, depend on them for protection rather 
than on fuses scattered about in fittings, rosettes, etc. Fig. 151 
shows common types of fused and unfused rosettes. 

BelERinging Transformers. Within the last few years small 
transformers have been brought out designed for the purpose of 
ringing door bells or for other light signaling work, deriving their 
power for such service directly from alternating-current lighting 





UNDERWRITERS’ REQUIREMENTS 


169 


circuits in houses. They take the form of small, totally enclosed 
transformers with two very thoroughly insulated coils, the primary 
coil to be connected directly across an alternating circuit and the 
secondary to be connected to the bell circuit as shown in Fig. 152. 

The push button is, of course, usually open so that no current 
flows over the bell circuit. When the button is pushed a current of 
low voltage (10 to 20 volts) and small ampere capacity (not over 2 
amperes) flows over the secondary or bell circuit, the power being 
derived from the primary winding connected to the alternating- 
current line as in the case of any transformer. Absolutely no de¬ 
pendence can be put upon the insulation of the bell circuit which is 
often of cotton - covered paraffined wire (annunciator wire) in¬ 
stalled in the most unreliable manner and the bells and push buttons 
are not designed for anything but very low voltage currents. It is. 



CIRCUIl TRANSFORMER 

Fig. 152. Diagram of Simple Transformer Circuit 

therefore, imperative that the design and construction and insula¬ 
tion of the transformer be such that under no conditions, either of 
service or from an accident, can the 110-volt current act directly on 
the bell circuit. Furthermore, the design of the transformer must 
be such that even if the push button be left closed or the bell wires 
become short-circuited only a very small current will flow over the 
bell wiring. In approved bell-ringing transformers both these results 
are secured with reasonable safety. 

Heating Devices. The rapid introduction of all sorts of electric 
heating devices for domestic and industrial use has brought with it 
a special hazard which it is peculiarly difficult to control. These 
devices are useful only as they are capable of developing a consider¬ 
able heat in a short time. The normal use, of course, tends to draw 
away the heat and thus prevent a dangerously high temperature 
being reached, but if these devices are left connected to the circuit 





















170 


UNDERWRITERS’ REQUIREMENTS 


and unused, many of them will reach a dull red heat and may thus 
become a serious fire hazard. This is true of electric flatirons espe¬ 
cially and also of many cooking utensils and other apparatus for 
domestic or light factory use. It is evident, therefore, that all such 
heating appliances should be made wholly of non-combustible ma¬ 
terial and, so far as their use will permit, be fitted with legs, guards, 
or other parts which will keep the heated parts well separated from 
walls, floors, tables, etc. 

Stationary heating devices should be installed only when every 
precaution has been taken and the fact that perhaps only a low 
temperature will be produced by the proper and normal operation of 
the heater should not be made an excuse for omitting any of the 
precautions that would be taken for the higher temperatures which 
may easily result from accident or misuse. There should be ample 
air spaces and proper protection of adjacent surfaces by asbestos 
board and metal sheathing. In general all electric heating devices 
must be installed and used as possible sources of great heat. 

Portable heating devices are not easily protected from misuse 
or accident. The chief protection against fires from such appliances 
appears to depend upon the original excellence of design and con¬ 
struction of the devices themselves, the fact that many of them 
employ but a small amount of energy, and, finally, upon a slowly 
growing appreciation by users and the public generally, that elec¬ 
trically-heated appliances, while usually fairly safe, if properly used, 
may very readily become dangerous if abused or improperly used. 

Electric Gas Lighters. A battery, spark coil, and similar 
appliances are often used for the purpose of lighting the gas on gas 
fixtures without the use of matches. In such installations the wires 
from the battery and coil are led to the fixtures in any convenient 
manner and on the fixtures themselves small wfires are carried down 
the outside of fixture stems and arms to the burners. The line wires 
are not insulated or installed in a manner comparable as to safety 
with electric light wires, and on the fixtures the insulation is espe¬ 
cially weak and exposed to injury. It is, therefore, evident that such 
gas-lighting systems should never be installed on the same fixture 
with electric lights, since a breakdown is very liable to permit the 
electric-light current to pass over the gas-lighting wires and cause 
a fire at some point perhaps concealed in a partition, either from over- 


UNDERWRITERS’ REQUIREMENTS 


171 


heating the small wires or from arcing to other wires or to grounded 
metal piping. 

Marine Work. The Code contains special rules governing the 
installation of electric light and power wires and apparatus on ship¬ 
board. These differ from the standard rules in only a few partic¬ 
ulars, as indicated by the need of special care to provide against 
the effects of constant and severe vibration, dampness and extreme 
hard usage to which marine installations are always subjected. 
The provisions of the Code should be referred to in detail by those 
who are called upon to install or inspect work of this special type. 
























' 


































































































































































































































■ 























































































































































































































































■ 






*V 








* 








INDEX 






























































































































‘ 















































































<- 



INDEX 


A PAGE 

Arc lamps. 88 

Armored cable. Ill 

B 

Bell-ringing transformers. 168 

Borders. 130 

C 

Car wiring. 136 

Cartridge fuses. 157 

Circuit breakers. 163 

Circuits, grounding of. 53 

Conduit fittings. 152 

Conduit work. 112 

Current systems (constant). 64 

Cut-outs. 154 

knife switches. 159 

snap switches. 161 

D 

Devices (miscellaneous). 165 

bell-ringing transformers. 168 

electric gas lighters. 170 

heating devices. 169 

marine work. 171 

panel boards and cabinets. 165 

rosettes. 168 

sockets and receptacles. 166 

Devices and materials. 148 

circuit breakers. 163 

fuses or cut-outs. 154 

miscellaneous. 165 

rigid conduit and conduit fittings. 152 

rubber-covered wire. 149 

Dimmers. 129 

E 

Electric gas lighters. 170 

Electric heaters. 77 

Electric signs. 126 

* Electrolysis. 49 




































2 


INDEX 


P PAGE 

Fixture wire. 151 

Fixture wiring. 80 

Fixtures. 80 

Flexible cords. 85 

Footlights.. v . 130 

Fuses.71, 154 

cartridge. 157 

plug. 156 

G 

Generators. 21 

Ground detectors and tests. 30 

H 

High- and extra high-potential systems. 137 

requirements for safety. 138 

I 

Inside work. 58 

Installation rules. 68 

constant-current systems. 64 

constant-potential systems. 67 

fixtures and fixture wiring. 80 

arc lamps on constant-potential circuits. 88 

fixture detail. 80 

flexible cords.'. 85 

sockets and receptacles. 82 

general rules for controlling and protecting devices. 68 

electric heaters. 77 

fuses and circuit breakers. 71 

switches. 68 

transformers in building. 90 

wiring systems.. 59 

Installation of wires in buildings. 91 

armored cable. Ill 

classification and general principles. 92 

concealed work. 104 

conduit work. 112 

open wiring in damp places. 97 

open wiring in dry places. 93 

wires in molding. 101 

K 

Knife switches. 159 

L 

Lighting (decorative). 124 

Lighting and power from railway wires. 137 

Lightning arresters. 28 

Lines (high tension). 51 










































INDEX 


3 


M PAGE 

Machines (moving picture). 133 

Marine work. 171 

Motors. '31 

Moving picture theaters and machines. 133 

causes of danger. 134 

interior equipment. 133 

N 

National electrical code. 19 

O 

Outside work. 43 

electrolysis. 49 

grounding of circuits. 53 

high tension lines. 51 

mounting of transformers. 53 

wiring. 44 

P 

Panel boards. 165 

Plug fuses. 156 

Potential systems (constant). 67 

Power stations and their equipment. 20 

generators. 21 

ground detectors and tests. 30 

lightning arresters. 28 

motors. 31 

resistance boxes or rheostats. .. *. 28 

storage batteries. 41 

switchboards. 25 

transformers. . .. . . 43 

R 

Receptacles.:.82, 166 

Resistance boxes or rheostats. 28 

Rigid conduit and conduit fittings.. . 152 

fittings. 153 

unlined steel. 152 

Rosettes. 168 

Rubber-covered wire. 149 

fixture wire. 151 

insulation for conduit and armored circuits. 152 

special insulation. 151 

S 

Signaling systems. 140 

protecting devices. 143 

wiring requirements. 140 

Snap switches. 161 

Sockets.82, 166 










































4 INDEX 

PAGE 

Special installations. 124 

car wiring. 136 

decorative lighting. 124 

electric signs. 126 

high- and extra- high-potential systems. 137 

lighting and power from railway wires. 137 

moving picture theaters and machines. 133 

signaling systems. 140 

theater wiring... 128 

Stage pockets. 131 

Steel conduit (unlined). 152 

Storage batteries. 41 

Switchboards. 25 

Switches. 68 


T 

Table 

conductors in alternating current, sizes of. 35 

conductors in direct current, sizes of. 34 

conduit, minimum weights of, for required wall thickness 153 

fuse spacing, open-link. 155 

knife switches, approved spacing for. 160 

rubber insulation, thickness of. 150 

wires, carrying capacity of. 65 

Testing of electric wiring equipment. 144 

voltmeter method. 145 

Theater wiring. 128 

dimmers. 129 

footlights and borders. 130 

general specifications. 128 

requirements for stage auditoriums. 133 

special lighting circuits and stage effects. 133 

stage pockets. 131 

Theaters (moving picture). 133 

Transformers.43, 90 

mounting of. 53 

U 

Underwriters’ requirements.1-171 

devices and materials. 148 

electric installations, essential parts of. 17 

electricity as cause of fires. 3 

elementary electrical ideas and terms. 4 

inside work. 58 

installation of wires in buildings. 91 

national electrical code. 19 

outside work. 43 

power stations and their equipment. 20 

special installations. 124 

testing electric wiring. 144 













































INDEX 


5 


y PAGE 

Voltmeter method for testing. 144 

W 

Wires 

general rules On. 62 

installation of, in buildings. 91 

in molding. 101 

Wiring. 44 

Wiring systems. 59 













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The School Behind the Book 


T HIS practical handbook is one of the representatives of 
the American School of Correspondence. It is the only 
kind of representative by which the School reaches the 
general public and extends its educational work. 

The American School of Correspondence is chartered, under 
the same laws as a State University, as an educational institution. 
Its instruction books, written especially to suit the needs of men 
seeking self improvement through correspondence work, are 
reserved for its students and for class use in educational institu¬ 
tions; many of these texts are used in the class room work of the 
best resident schools in the country. 

However, in order that the large number of ambitious men, 
for whom class work and correspondence study are neither prac¬ 
tical nor advisable, may not be deprived of this valuable material, 
it is published by the School both in sets covering the several 
branches that it teaches, and in a series of single Home Study 
volumes treating of specialized lines cf practical knowledge. This 
book is a sample of the make-up of the Home Study volumes and 
the titles and authors are shown on the following page. By this 
method the School broadens its field of activity; and from these 
sales it derives an income to use in general educational work. 

The School's publications are clear and practical, and will 
be found ideal for reference and home reading. For those, how¬ 
ever, who desire more systematic study of the subjects in which 
they are particularly interested, the School advises a thorough 
course by correspondence as the quickest and surest means of 
obtaining the practical knowledge desired. 

The School offers correspondence instruction in all branches 
of architecture, civil engineering, college preparatory work, account¬ 
ing and business administration, drawing and design, electrical 
engineering, fire prevention and insurance, American law, mechan¬ 
ical, sanitary, and steam engineering, and textile manufacturing. 
It adapts its courses to the needs of the individual, by starting him 
where his previous education stopped, and giving him only such 
work as is necessary to fit him for the work he wants to do. 

On request the School will mail to any address a Bulletin 
containing full information regarding its courses and methods. 
It employs no representative other than its own publications. 

AMERICAN SCHOOL OF CORRESPONDENCE 

CHICAGO, U. S. A. 





American School of Correspondence 

PRACTICAL HANDBOOKS FOR HOME STUDY 


O WING to a constant and increasing demand for 
low-priced single volumes covering the sub¬ 
jects treated in the courses and cyclopedias 
of the American School of Correspondence, a 
series of practical handbooks have been com¬ 
piled to be sold through the Book Stores all over the 
world. If any purchaser finds that his local dealer does 
not carry the particular title which interests him, he 
can order direct from the publisher, who will make 
shipment on receipt of price. If, after five days’ exam¬ 
ination, the volume is found unsuited to his need, the 
purchaser may return it and his money will be promptly 
refunded. 


Partial List of Titles and Authors 


Alternating-Current Machinery_William Esty_$3.00 

Architectural Drawing and Lettering_Bourne-von Holst-Brown 1.50 

Bank Bookkeeping_Charles A. Sweetland_1.00 

Boiler Accessories_Walter S. Leland_1.00 

Bridge Engineering—Roof Trusses_Frank O. Dufour_3.00 

Building and Flying an Aeroplane__Charles B. Hayward_1.00 

Building Superintendence_Edward Nichols_ 1.50 

Business Management, Part I_James B. Griffith_1.50 

Business Management, Part II_Russell-Griffith_ 1.50 

Carpentry_Gilbert Townsend_1.53 

Care and Operation of Automobiles_Morris A. Hall_1.00 

Commercial Law_John A. Chamberlain_3.00 

Compressed Air_Lucius I. Wightman_1.00 

Contracts and Specifications_ _James C. Plant_ 1.00 

Corporation Accounts and the Voucher System_ .James B. Griffith_ 1.00 

Cotton Spinning_Charles C. Hedrick_3.00 

Department Store Accounts_Charles A. Sweetland_1.50 

Descriptive Astronomy_Forest Ray Moulton_1.50 

Dynamo-Electric Machinery_F. B. Crocker_ 1.50 

Electric Railways_ Henry H. Norris_1.53 

The Electric Telegraph Thom-Collins_ 1.00 







































Partial List of Titles and Authors — Continued 


Electric Wiring and Lighting_ 

Estimating_ 

Factory Accounts_ 

Forging- 

Foundry Work_ 

Freehand and Perspective Drawing. 

The Gasoline Automobile_ 

Gas Engines and Producers_ 

Heating and Ventilation_ 

Highway Construction_ 

Hydraulic Engineering_ 

Insurance and Real Estate Accounts 

Knitting_ 

Machine Design_ 

Machine-Shop Work__ 

Masonry and Reinforced Concrete _ 

Masonry Construction_ 

Mechanical Drawing- 

Modern American Homes_ 

Motion Pictures_ 

The Orders_ 

Pattern Making_ 

Plumbing_ 

Power Stations and Transmission. _ 

Practical Aeronautics_ 

Practical Bookkeeping- 

Practical Lessons in Electricity- 

Reinforced Concrete- 

Railroad Engineering- 

Refrigeration- 

Sewers and Drains- 

Sheet Metal Work_ 

Stair-Building and Steel Square. _. 

Steam Boilers_ 

Steam Engines- 

Steam Turbines-1- 

Steel Construction_ 

Strength of Materials- 

Surveying_ 

Telephony_ 

Textile Chemistry and Dyeing- 

Textile Design_ 

Tool Making_ 

Valve Gears and Indicators- 

Water Supply- 

Weaving_ 

Wireless Telegraphy and Telephony 

Woolen and Worsted Finishing- 

Woolen and Worsted Spinning- 


PRICE 

Knox-Shaad_$1.00 

Edward Nichols_ 1.00 

Hathaway-Griffith_1.50 

John Lord Bacon_1.C0 

Wm. C. Stimpson_.. 1.00 

Everett-Lawrence_1.00 

Lougheed-Hall_2.00 

Marks-Wyer_ 1.00 

Charles L. Hubbard_ 1.50 

Phillips-Byrne_1.00 

Turneaure-Black_3.00 

Charles A. Sweetland_1.50 

M. A. Metcalf_3.00 

Charles L. Griffin_ _ 1.50 

Frederick W. Turner_1.50 

AVebb-Gibson_ . 3.00 

Phillips-Byrne_ 1.00 

Ervin Kenison_ 1.00 

H. V. von Holst_ _ 3.00 

David S. Huffish_4.CO 

Bourne-von Holst-Brown 3.00 

James Ritchey_ 1.00 

Gray-Ball_ 1.50 

.Geo. C. Shaad_ 1.00 

Chas. B. Hayward_ 3.50 

James B. Griffith_1.50 

. Millikan-Knox-Crocker _ 1.50 

AVebb-Gibson_ 1.00 

Walter Loring Webb_3.CO 

. M. W. Arrowwood_1.00 

A. Marston_ 1.00 

William Neubecker_3.00 

Hodgson-Williams_1.C0 

Newell-Dow_ 1.00 

L. V. Ludy_ 1.00 

Walter S. Leland_1.00 

E. A. Tucker_ 1.50 

Edward Rose Maurer_1.00 

Alfred E. Phillips_ 1.50 

Miller-McMeen_4.00 

Louis A. Olney--3.00 

Fenwick Umpleby_3.00 

Edward R. Markham_1.50 

L. V. Ludy_ 1.00 

Frederick E. Turneaure_ _ 1.00 

H. William Nelson_ 3.00 

Ashley-Hayward_1.00 

John F. Timmerman_3.00 

Miles Collins_3.00 



























































































JUN 18 1918 












































































































































































































































































































































